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Diese Studie wendet einen erweiterten Aquifer Vulnerability Index auf Black Hawk County, Iowa, an, der sich auf vier Schlüsselbereiche konzentriert: Grundwasseranreicherung, Reisezeit zu den Aquiferen, Verschmutzungsrisiken und Grundwassernutzung. Die Forschung identifiziert Regionen mit höherer Anfälligkeit, insbesondere in Überschwemmungsgebieten und Gebieten mit flachem Untergrund, wo schnellere Grundwassertransportzeiten und erhöhte Verschmutzungsrisiken vorliegen. Die Studie unterstreicht auch die schützende Rolle unoxidierter Gletscherbestände bei der Verringerung der Anfälligkeit von Grundwasserleitern. Durch Einbeziehung dieser Faktoren bietet der erweiterte Index eine umfassendere Bewertung der Anfälligkeit von Grundwasserleitern, die durch Nitratkonzentrationsdaten aus öffentlichen und privaten Brunnen validiert wird. Dieser Ansatz bietet wertvolle Erkenntnisse für die Lenkung der Landnutzung und Ressourcenplanung zum Schutz wertvoller Grundwasservorräte.
KI-Generiert
Diese Zusammenfassung des Fachinhalts wurde mit Hilfe von KI generiert.
Abstract
Vulnerability assessments are frequently used to guide land use and resource planning efforts in groundwater-dependent regions. However, many readily available methodologies are not particularly useful to assess pollution risks to aquifers in specific hydrogeologic settings. In this study, aquifer vulnerability was evaluated in Black Hawk County, Iowa (USA), using an index-based approach conceptually aligned with traditional methodologies focused on groundwater resistance through overlying sedimentary layers. Incorporating elements of groundwater recharge, travel time to the uppermost used aquifer, pollution risk and groundwater use, the enhanced aquifer vulnerability index (AVI) method revealed spatial variability in vulnerability at local and regional scales within the county. Increased aquifer vulnerability was evident within the Cedar River floodplain and other minor floodplain areas where there is greater recharge, shorter vertical groundwater travel times, the presence of point and nonpoint pollution sources, and increased water use. In upland regions underlain by unoxidized till, bedrock aquifers are largely protected from pollution, whereas in areas where the bedrock surface is relatively shallow, the protection of unoxidized till is missing and aquifer vulnerability to contamination is higher. Groundwater use reflected by incorporating 10-year capture zones into the index model identified zones where water supply aquifers are at greater risk from point and nonpoint source contamination. Overall, the new methodology adapts and greatly expands on the traditional AVI approach and can be adopted in other regions as applicable. Users are able to modify and adapt the index-based groundwater vulnerability schemes to better account for localized patterns and location-specific use and pollution risks.
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Introduction
Groundwater supplies are vital for meeting current and future demands for human consumption, food production, energy, and the environment, but long-term sustainability is threatened by over use and increasing pollution risks (Gorelick and Zheng 2015). Groundwater accounts for approximately one-third of freshwater withdrawals worldwide (Famiglietti 2014) and provides nearly 40% of the public water supply and 98% of domestic water in the United States (Maupin et al. 2014). In an agricultural state such as Iowa, located in the US Midwest, surface water bodies are often impaired by excessive nutrient pollution (Jones et al. 2018; Schilling and Wolter 2009) and groundwater provides nearly 80% of public water supply use (Prior et al. 2003). Identification and protection of vulnerable groundwater supplies from potential contamination is needed to ensure water is available for future generations (Gleeson et al. 2012).
Groundwater or aquifer vulnerability assessments have received considerable attention in the literature, including several comprehensive reviews of conceptual approaches and comparisons of methodologies (e.g., Taghavi et al. 2022; Fannakh and Farsang 2022; Machiwal et al. 2018; Wachniew et al. 2016; Goyal et al. 2021; Brindha and Elango 2015). Reviews often distinguish between index-based approaches (GIS-based qualitative approaches), quantitative process-based methods that involve detailed modeling, as well as statistical-hybrid models (see Taghavi et al. 2022). They further subdivide both approaches for applicability to porous media or karst aquifers (Machiwal et al. 2018). With respect to index-based approaches applied to porous (non-karst) aquifers, three methods are routinely highlighted: DRASTIC (Aller and Thornhill 1987) and DRASTIC derivatives (see Brindha and Elango 2015); GOD (Duijvenbooden and Waegeningen 1987) and AVI (Van Stempvoort et al. 1993).
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Among these approaches (as well as others), there are strengths, weaknesses, and data considerations that make the choice of an appropriate method dependent on user goals and objectives. One distinguishing difference among several methods is whether the user is interested in assessing groundwater vulnerability at the top of the water table or vulnerability to deeper aquifers. For example, DRASTIC includes terms related to depth to water table, soil media, and vadose zone thickness that focus on risks of surface contamination reaching the water table by infiltration (Aller and Thornhill 1987). In contrast, a method such as AVI focuses on groundwater vulnerability based on the thickness of sedimentary units above an aquifer and hydraulic conductivity (K) of these units (Van Stempvoort et al. 1993; Wachniew et al. 2016). This method provides an approximate travel time for water to move downward through different layers to the aquifer. In Iowa and other glaciated Midwestern US states where the water table in upland areas resides in fine-textured glacial till with extremely low K (Rodvang and Simpkins 2001), groundwater vulnerability to the water table does not equate with potential aquifer use. Hence, the DRASTIC index and similar formulations may not be particularly useful to assess pollution risks to aquifers that people are using for water supply. Rather, an AVI-based approach that considers the travel time to an aquifer may be more appropriate.
In this study, aquifer vulnerability in Black Hawk County, Iowa, was assessed using an index-based approach loosely aligned with traditional AVI methodology. In this formulation, additional elements were added to the travel time concept, which included new features related to groundwater recharge, contaminant risks and groundwater use to more holistically assess aquifer vulnerability in the region. Using geologic mapping information completed for the county (Tassier-Surine et al. 2013; Rowden et al. 2013) and available high-resolution geospatial data, the approach essentially follows one-dimensional (1-D) transport of groundwater from recharge at the water table to downward travel through confining beds into aquifers used by local residents. The contaminant risks to groundwater from point and nonpoint source pollution and groundwater capture for drinking water use by public and private wells. The goal of the new aquifer vulnerability index for Black Hawk County is to provide a map product useful for guiding land use and resource planning that would be protective of valuable groundwater aquifers.
The objectives of this paper are to: (1) describe the enhanced AVI methodology with specific application to Black Hawk County, Iowa; (2) assess aquifer vulnerability at a 30 m resolution in the county and identify areas of greater risk for contamination; and (3) discuss the usefulness and applicability of the methodology to other regions and opportunities for further development of index-based approaches. Overall, the approach uniquely expands on a traditional AVI concept initially proposed by Van Stempvoort et al. (1993) to include additional factors that describe groundwater recharge, pollution sources, and groundwater use in the county. Expanding on the traditional approach with additional local-specific factors greatly increases the utility of the assessment to guide local land management decisions.
Methods
Regional setting
Black Hawk County is an area of 1470 km2 located in northeast Iowa (Fig. 1). The county lies within the Iowan Erosion Surface (IES) Landform Region (Prior and Kohrt 2006), a gently rolling landscape formed by multiple periods of Quaternary glaciations and post-glacial subaerial erosion. The county is covered by Quaternary deposits consisting of loamy colluvial sediments of variable thickness overlying Pre-Illinoian glacial deposits (Tassier-Surine et al. 2013). Pre-Illinoian tills were deposited between approximately 2600 to 500 ka in eastern Iowa and consist of dense (bulk densities are 1.76–2.11 Mg/m3; Kemmis et al. 1992) and loamy sediments (30–50% sand, 30–45% silt, and 20–25% clay; Hallberg 1980) with a very low hydraulic conductivity (K = 1.2 × 10–7 m/s; Schilling and Tassier-Surine 2006). The Cedar River valley that bisects the county is filled with Wisconsin-age fluvial and glaciofluvial sand and gravel deposits that are often mantled by younger Holocene terrace materials. Quaternary deposits have a maximum thickness of up to 73 m (240 ft) in the bedrock valleys (Tassier-Surine et al. 2013).
Fig. 1
Location of Black Hawk County and major geographic features
Quaternary materials are underlain by Devonian and Silurian carbonate bedrock (Rowden et al. 2013). Devonian rocks are dominated by carbonates varying between limestone and dolostone with accompanying minor shale, whereas the Silurian is comprised mostly of dolostone with varying amounts of chert and minor shale. Both rock systems are typically combined together as a single bedrock aquifer in the region (Silurian-Devonian aquifer). Elsewhere in the county, Ordovician Maquoketa Formation rocks occur in the deepest portions of the bedrock valleys in the northeast part of the county and include an interbedded green to gray dolomitic shale and shaly dolostone with minor limestone.
The population of Black Hawk County is approximately 131,000 with most residents residing in the cities of Waterloo (67,000) and Cedar Falls (41,000) located on the Cedar River floodplain (Fig. 1). Both Waterloo and Cedar Falls rely on a combination of alluvial and Silurian-Devonian aquifer bedrock wells for their water supply, whereas other communities and most private wells in the county primarily tap into the upper Silurian-Devonian bedrock aquifer.
Enhanced aquifer vulnerability index model
The original AVI method described in Van Stempvoort et al. (1993) reported aquifer vulnerability based on two physical parameters: (1) the thickness of each sedimentary layer above the uppermost saturated aquifer; and (2) the K of the sedimentary layers. Using these two layers, the hydraulic resistance to vertical groundwater flow could be calculated. The resistance term is roughly equivalent to travel time but does not include stratigraphically explicit hydraulic gradients or porosity terms in the calculation. Further, the method does not consider groundwater recharge or account for factors that contribute to aquifer risk from contamination or whether or not the aquifer is being used.
In the expanded approach to the AVI, four main geospatial layers were compiled at a 30 m resolution in the county consisting of: (1) groundwater recharge; (2) groundwater travel time to the uppermost aquifer; (3) contaminant risk from point and nonpoint sources; and (4) groundwater use (Fig. 2). Each of these factors were scored for every grid cell in the county and the accumulated total score for a cell provided an estimate of aquifer vulnerability. A description of the model layers and scoring assumptions are provided in Table 1 and discussed in detail below. A flow chart of the geospatial inputs used to create the model layers is shown in Fig. 2. Existing input information was used to create some model layers but new derivative data was developed from this study to create the depth to first aquifer and travel time layers.
Fig. 2
Workflow showing input information and model layers needed for the groundwater vulnerability map
Summary of vulnerability factors, scoring weights, and justification
Vulnerability factor
Total factor score
Scoring weights
Justification
Groundwater recharge
10
10 is cell is located on floodplain;
5 if cell is located in upland;
2 if cell is located on sideslope
Scoring consistent with relative proportion of recharge in Iowa based on landscape patterns (see Schilling et al. 2018)
Travel time to aquifer
20
20 if travel time < 10 years;
15 if travel time 10–50 years;
10 if travel time 50–100 years;
5 if travel time 100–500 years;
0 if travel time > 500 years
Maximum of 20 points due to overriding importance compared to other factors
Scoring evenly divided based on range of possible travel times found for county
Higher scores associated with faster groundwater travel times: score of 20 if geology dominated by high K materials; score of 15 (< 50 years) consistent with tritium-based groundwater age (see Schilling and Tassier-Surine 2006); scores 10–0 gradual reduction in risk with increasing travel time
Groundwater risk
10
10 if known point source is located in a cell;
5 if cell is overlain by row crop land cover;
0 if cell contains no point sources or is overlain by perennial cover
Risk for groundwater pollution was assumed to be higher with potential point source within cell (score of 10)
Row crop land cover significantly related to river nitrate concentration (Schilling and Libra 2000)
Perennial cover assumed to be unfertilized with limited nitrate leaching risk
Groundwater use
10
10 if cell is located within 10-year capture zone for public water supply well or cell contains a known private well in IGS database;
0 if cell is located outside of capture zone or does not contain a known private well
Binary variable (10 or 0) based on whether groundwater is being used for water supply
Capture zones include areas contributing groundwater to pumping wells within 10 years
Private wells with limited pumping and zone of influence assumed to be within extent of single cell
Score of 0 if no known aquifer use in cell
Total maximum score
50
-
Maximum score is sum of factor scores
Groundwater recharge
Groundwater recharge occurs when infiltrating water enters the water table surface, and it is often dependent on many factors; indeed, the DRASTIC method includes several thematic layers that relate to water transport through soils, such as depth to water table, net recharge, soil media, topography, and impact of vadose zone (e.g., Aller and Thornhill 1987; Brindha and Elango 2015). However, many of these soil factors are related and spatially overlap and therefore may provide redundant information in a groundwater vulnerability index. In this Iowa-based example, soils are overwhelmingly fine-textured in loess-mantled uplands and coarser-textured in lowland areas, including floodplains. Hence, a landscape approach to recharge that captures the inherent spatial patterns in soil texture provides a suitable surrogate for groundwater recharge without adding additional terms or redundant information.
Previous work in Iowa established that more groundwater recharge occurs in the floodplain, followed by flat uplands (< 5% slope) and then on sideslopes (Schilling 2009; Schilling et al. 2018). In Clear Creek watershed in eastern Iowa, the floodplain received 404 mm of recharge compared to 211 mm in the uplands, or 37% and 26% of annual precipitation, respectively (Schilling et al. 2018). In Walnut Creek (south-central Iowa) floodplain and upland recharge averaged 44 and 24% of annual precipitation, whereas recharge in sideslopes was substantially less (14% of annual precipitation; Schilling 2009). Overall, a greater fraction of precipitation is converted to recharge in floodplains where the water table is shallow and soil specific yield is higher (Sumner 2007). In contrast, in the upland tills, infiltrating precipitation is captured in unsaturated soils rather than reaching the water table and groundwater recharge only occurs after soil-moisture reserves are filled (Schilling 2009).
The SSURGO soils coverage was used to determine the distribution of landscape positions in the county. Floodplain polygons were selected in the GeoDesc field, whereas soils with a slope value of ≤ 5 and > 5% were selected to determine uplands and sideslopes, respectively. Using a 10-point scoring system for the layer, cells located within floodplains were given a maximum recharge score of 10, upland cells were given a score of 5 and sideslope cells were given a score of 2. These scores reflected the relative proportions of annual precipitation recharged to the water table based on landscape position.
Travel time to aquifer
Groundwater travel time to the aquifer was evaluated using a 1-D approach that considered geologic stratigraphy and application of Darcy’s Law to estimate vertical groundwater flow velocity. Principal aquifers being used in Black Hawk County were first identified and mapped to determine the uppermost aquifer used by county residents (Fig. 3). Municipal and private well users in the county primarily utilized either the alluvial sands and gravels along major rivers or the uppermost bedrock unit for their water supply (mainly Devonian Coralville and Little Cedar formations). Lithologic logs were obtained from the Iowa Geological Survey’s Geosam database (IGS 2025) to map spatial patterns in aquifer availability within the county at a 30-m spatial resolution. It is important to note that the uppermost aquifer was not aquifer-specific but varied on the basis of the vertical stratigraphy encountered at a location (Fig. 3).
Fig. 3
Generalized stratigraphy of Black Hawk County. The first available aquifer encountered in a cell varied according to local stratigraphy
Using the lithologic log database (Geosam), map layers of Quaternary stratigraphic units from the land surface to uppermost aquifer were delineated at a 30 m resolution. Stratigraphic layers consisted of alluvial sand and gravel (modern alluvium), glacial outwash (coarse sand and gravel), oxidized pre-Illinoian till, interbedded sand and gravel within the till, and unoxidized pre-Illinoian till. Not all of these layers were present at all locations, so the distribution of stratigraphic map layers varied within the county. In upland regions with thick deposits of glacial materials, the stratigraphic sequence was often comprised of oxidized till overlying unoxidized till, with variable occurrences of interbedded sand and gravel deposits. In the floodplains, the stratigraphic sequence was typically modern alluvium overlying glacial outwash alluvium (Fig. 3).
Groundwater travel time to the aquifer was evaluated using application of Darcy’s Law to estimate vertical groundwater flow velocity. An average hydraulic conductivity was assigned to each unit based on previous work in Iowa (Schilling and Tassier-Surine 2006; Schaap et al. 2003; Haj et al. 2021). Groundwater flow and velocity through pre-Illinoian till was investigated by Schilling and Tassier-Surine (2006) at a research site in Linn County, located approximately 60 km south of Black Hawk County. An 8- and 6-well monitoring well nest installed into a 31-m thick pre-Illinoian till sequence was characterized using traditional hydrologic methods and chemical tracers. The aquitard system consists of about 9 m of fine-grained oxidized pre-Illinoian till overlying 22 m of unoxidized till and Devonian dolomite bedrock. K in oxidized till averaged 1.2 × 10–7 m/s and averaged 4.5 × 10–10 m/s in unoxidized till. An interbedded sand unit at the till hydrology site had a K of 2 × 10–6 m/s. Hydraulic head relations among the nested wells indicated downward groundwater flow through the till profile. The downward vertical hydraulic gradient within the oxidized till unit was 0.03 and the gradient increased to 1.2 through the unoxidized till. Hydraulic gradients were steepest near the unoxidized till/bedrock interface. The porosity of Iowa till units estimated by Helmke et al. (2004) ranged from 28.6 to 31.2% and assumed a value of 30% for the pre-Illinoian till in this study.
Hydrologic studies of the Cedar River alluvium in the Cedar Falls area (Schaap et al. 2003) and in the Cedar Rapids area located approximately 80 km downstream (Haj et al. 2021) were used to estimate K of the modern and glacial outwash alluvium. The K of the modern alluvium averaged approximately 3.5 × 10–5 m/s, whereas K averaged 1.7 × 10–4 m/s in the coarser textured outwash. Although both the modern and outwash alluvium had a large range in measured K values, the average value used provided sufficient differentiation for travel time classifications (a higher K would still place the cell in the lowest travel time category; i.e., less than 10 years). The vertical gradients in the alluvium and outwash were assumed to be very low (most flow is horizontal in permeable aquifers) and averaged 0.1% for both aquifer systems. The porosity was assumed to be 30% (Freeze and Cherry 1979).
The average linear velocity (V) of groundwater through stratigraphic sequence was calculated using Darcy’s Law:
$$\textbf{V}=-Ki/n$$
(1)
where K and i are hydraulic conductivity and the hydraulic gradient of individual layers, respectively, and n is porosity. Vertical groundwater flow velocities calculated for each unit were multiplied by their respective unit thickness to produce a thickness-weighted 1-D travel time for each stratigraphic unit. The travel times by unit were added together to produce a groundwater travel time for the entire sequence from the ground surface to the uppermost aquifer. In this AVI formulation, travel times were weighted higher than other factors and scored for a maximum of 20 points based on five categories of time: 0–10 years = 20 points; 10–50 years = 15; 50–100 years = 10; 100–500 years = 5; and > 500 years = 0.
Risks from point and nonpoint source pollution
Risks for groundwater contamination by point and nonpoint sources were determined using available geospatial data. Point sources were located and mapped by the Iowa Department of Natural Resources (Iowa DNR) as part of the US EPA Source Water Protection (SWP) program. Point sources extracted from the IDNR database included the locations of leaking underground storage tanks, landfills, CERCLA sites, hazardous waste generators, and other potential sources. The nonpoint source risk to groundwater contamination was determined by the presence or absence of row crop agriculture in the 30 m grid cell. The National Land Cover Database (NLCD) was used to map the distribution of cultivated crops in the county. Row crops of corn and soybeans, along with added manure risks from manure applications to the same cropped areas, constitutes the largest nonpoint source risk in Iowa (Schilling and Libra 2000; Weldon and Hornbuckle 2006). Risks were assigned to a cell if they contained a point source (10 points) or contained row crop land cover (5 points). Cells with no point sources present or land cover in perennial vegetation were assigned a score of 0.
Groundwater use
Groundwater use was assessed by determining whether a cell location was within a 10-year time of travel capture zones for a public water supply well (as delineated by the Iowa DNR SWP program (IDNR 2025). The 10-year capture zones provide an estimate of the area of groundwater that will flow to a well within a 10-year period. Cells that contained a known private well within a private well tracking system database were also identified (IWIS 2025). The presence of a 30 m cell intersecting a capture zone or containing a known private well was used to assign a groundwater use factor. The groundwater use score for a cell was assigned either a value of 10 (cell contained a private well or was within a capture zone) or 0.
Final model
The scores for the four factors, including recharge (10 points), time of travel (20 points), groundwater risk from point and nonpoint source pollution (10 points), and groundwater use (10 points) were added together for each 30-m cell for a maximum score of 50. The spatial distribution of aquifer vulnerability by cell was subsequently mapped for Black Hawk County. Validation of model results was conducted by comparing the vulnerability score for a cell to measured nitrate concentrations in private and municipal wells in the county. Private well data were obtained from the Iowa Department of Natural Resources (IDNR) private well tracking system (IWIS 2025). Private wells were selected if they featured known well depth information that indicated the well was less than 30 m deep. Municipal data were also obtained from the IDNR from the Iowa drinking water portal (IWIS 2025). Municipal wells were selected for analyses if their well depths screened unconsolidated deposits or the Devonian Cedar Valley Formation. Municipal or private wells that were deeper than first available aquifer were not included in the validation.
It should be noted that the final model of aquifer vulnerability for Black Hawk County does not include karst-specific factors even though the bedrock aquifer is carbonate. Aquifer vulnerability in karst regions often includes features that contribute to rapid transport of surface contaminants to groundwater through conduits, such as sinkholes, losing streams, and other features of an active karst network (Ravbar and Goldscheider 2009). In the case of Black Hawk County, karst features have not been mapped in the area and groundwater transport to the bedrock aquifer occurs through glacial and fluvial materials.
Results
Individual factors and spatial patterns
In the gently rolling and weathered glacial landscape found in Black Hawk County, much of the landscape consists of relatively flat upland divides (slopes less than 5%) and the floodplains of streams and rivers, including the dominant floodplain of the Cedar River (Fig. 4). Slopes are primarily found along the valley edges of the dissected stream network. Regional groundwater recharge would be concentrated in the floodplains where the water table is shallow (typically less than 2 m; Schilling and Jacobson 2012) and unsaturated soils consist largely of sand. Soils in upland areas typically consist of silty loess overlying oxidized till and water tables often fluctuate between 3 and 5 m below ground surface (Schilling 2009). Overall, 25.5% of the county comprises floodplains with greater recharge, compared to uplands and slopes that comprise 67.1 and 7.4% of the county, respectively.
Fig. 4
Spatial distribution of groundwater recharge partitioned by landscape position
In the calculation and scoring of the time of travel time factor, a preliminary step was determining the depth to the first aquifer (Fig. 5). In Black Hawk County, principal aquifers include mainly the uppermost Silurian-Devonian bedrock or the alluvium of the Cedar River, with only scattered use of buried sand and gravel present within the glacial drift. Depth to first aquifer was typically less than 7.6–15.2 m (25–50 feet) in the floodplain corridors and in shallow bedrock areas east of the Cedar River floodplain. Regions with the greatest depth to aquifer, exceeding 61–91 m (200–300 feet), were associated with a bedrock channel that extends from the southwest portion of the county to the north and east and crosscut by the modern Cedar River (Fig. 5). Water supply and private wells still tap the Silurian-Devonian aquifer, but they draw water from a lower portion of the aquifer. Hence, there is considerably more glacial drift sitting above the aquifer in the bedrock channel areas.
Fig. 5
Depth to the first used aquifer in Black Hawk County
The time of travel to the aquifer was estimated for Black Hawk County (Fig. 6). As would be expected, vertical groundwater travel time in the floodplain areas was often less than 10 years, and less than 50 years in floodplain terrace regions. Likewise, in shallow rock areas, groundwater travel times were less than 50 to 100 years. In shallow rock areas, overlying sediments often consist of oxidized and fractured pre-Illinoian till that allows for more rapid water and contaminant transport (Helmke et al. 2004). In contrast, travel times in upland areas underlain by unoxidized glacial till exceeded 500 years (Fig. 6). The travel times associated with pre-Illinoian till in Black Hawk County are similar to values estimated for Linn County approximately 80 km south (Schilling and Tassier-Surine 2006). Maximum travel time scores of 20 are observed in the floodplain but are 0–5 in upland areas underlain by unoxidized till (Fig. 6).
Fig. 6
Estimated vertical groundwater travel time to the first used aquifer in Black Hawk County
The distributions of groundwater risks from point and nonpoint source pollution, and groundwater use are shown in Fig. 7. It is clear that much of the upland areas of the county are cropped under corn and soybean cultivation with associated nonpoint source pollution risks. In these areas, application of fertilizers and pesticides, and manure from animal feeding operations, can threaten groundwater quality. On the other hand, most of the point source risks were concentrated in the Waterloo-Cedar Falls metropolitan area and in smaller towns in the county (red circles; Fig. 7). Groundwater contamination from point sources can threaten groundwater quality over large geographic areas. Indeed, several plumes of chlorinated hydrocarbons and PFAS compounds from current and former manufacturing plants, dry cleaners, and abandoned disposal sites have been investigated in the Waterloo-Cedar Falls area (IDNR Contaminated Sites 2025). However, it is important to note that in this analysis, no differentiation was made between sites with identified leaks or plumes versus those sites that are regulated hazardous waste handlers or generators. In the scoring scheme, any cell with a potential point source was given a score of 10 points because this correlates with increased risk of groundwater pollution.
Fig. 7
Distribution of point sources and nonpoint source agricultural row crop land in Black Hawk County. Areal extent of 10-year capture zones and locations of private wells are also shown
With concentrated population in the Waterloo-Cedar Falls metropolitan area, water supply systems that capture groundwater for municipal and industrial use are also located in this area (Fig. 7). Most of the capture zones are circular in shape because detailed hydrogeologic studies have not been performed to precisely delineate the zones. The circular shapes of many capture zones, as delineated by state regulators, were based primarily on the estimated hydraulic conductivity of the aquifer and the pumping rates for the wells. Larger capture zones are associated with aquifers with greater K and pumping rates. In some cases, capture zones were truncated at major rivers when the rivers were considered as a hydraulic barrier. Despite inaccuracies in capture zone delineations, greater groundwater use can be expected within these areas compared to areas without capture zones. In rural areas, nearly all farmsteads have a well but pumping rates by these wells are not particularly large. For private well sites, it was assumed that the capture zone was essentially the 30-m cell that contained the well.
Final aquifer vulnerability model
The aquifer vulnerability map for Black Hawk County shows that vulnerabilities within the county are not evenly distributed and that some areas have greater potential for impacts than others (Fig. 8; map sheets are available at Schilling et al. 2024). Increased aquifer vulnerability was evident within the Cedar River floodplain and other minor floodplain areas where there is greater recharge, faster vertical groundwater travel times, the presence of point and nonpoint sources risks, and the cell occurs within a groundwater capture zone. In contrast, other regions of the county underlain by unoxidized till are largely protected from groundwater risks. These non-floodplain and primarily rural areas tend to have widespread risks from nonpoint sources but there is little large-scale pumping of bedrock aquifers (indicated by lack of capture zones). Bedrock aquifers are largely protected by the glacial overburden. However, in areas where the bedrock surface is relatively shallow, the protection of unoxidized till is often missing and aquifer vulnerability to contamination is higher.
Fig. 8
Aquifer vulnerability map for Black Hawk County. Total index score for a 30-m cell was based on recharge (maximum 10 points), travel time (20 points), pollution sources (10 points), and groundwater use (10 points)
The spatial resolution of aquifer vulnerability is highlighted in an urbanized region of central Black Hawk County (Fig. 9). Within this area, a large spectrum of vulnerabilities is shown, ranging from highest vulnerability within capture zones on the floodplain of the Cedar River to protected upland locations underlain by glacial till. There is a clear influence of the circular capture zones on the spatial distribution of aquifer vulnerability, but it is evident that vulnerability within a single capture zone can vary considerably, often controlled by landscape position (upland or floodplain), hydrogeology, and point source risks.
Fig. 9
Enlarged area of aquifer vulnerability map showing the spatial resolution of vulnerability mapping available for the county
Validation of the aquifer vulnerability approach was conducted by comparing vulnerability scores to groundwater nitrate concentrations measured in public water systems and private wells (Fig. 10). Overall there were a total of 233 private wells and 139 municipal water supply wells that met criteria for inclusion in the validation (372 total). Nitrate concentrations greater than 3 mg/l were used as a cutoff value indicative of anthropogenic nitrate derived from land surface activities (Jones et al. 2020). Mapping results indicated a close relationship between vulnerable aquifer regions and the absence of substantial thickness of unoxidized pre-Illinoian till (Fig. 10). Nitrate concentrations > 3 mg/l were clustered in floodplain regions with little or no unoxidized till cover. Of the population of 372 private and public wells in the county that tap the first available aquifer, 266 wells were located in a cell with an aquifer vulnerability score greater than 20 (71.5%).
Fig. 10
Locations of private and municipal wells in Black Hawk County with measured nitrate concentrations that are either less than 100 feet deep (private wells) or tap into unconsolidated and upper Devonian bedrock aquifers (municipal). Approximately 71.5% of the wells with nitrate concentrations > 3 mg/l are located in a cell with a vulnerability score > 20
In this application of an enhanced aquifer vulnerability index, the central idea from the AVI method of Van Stempvoort et al. (1993) was retained, that is, to assess vulnerability to aquifers based on the thickness and K of sedimentary layers between the ground surface and aquifer, and expanded the analysis to include groundwater recharge, pollution risk and groundwater use. Further, groundwater travel time was explicitly accounted for by estimating average linear velocities through various aquitard layers rather than quantifying “hydraulic resistance” as done by Van Stempvoort et al. (1993). Using this new approach, groundwater flow pathways were essentially followed from recharge to the water table downward through geologic layers into aquifers that are being used by water systems and private well owners. By accounting for risks due to nonpoint and point source pollution, aquifer vulnerability was further assessed by including the proximity to known contaminant sources.
While the methodology expands on the traditional AVI methodology, it simplifies some of the features utilized in the popular DRASTIC method and other DRASTIC-based derivative methods (see Aller and Thornhill 1987; Brinda and Elango 2015; Fannakh and Farsang 2022). For example, the DRASTIC method includes factors such as depth to water level, soil media, and impact to vadose zone that describe groundwater vulnerability from surface migration of contaminants to the water table. The approach focused on aquifer vulnerability, whereby factors associated with the soil zone and shallow groundwater above the aquifer are captured by other inclusive factors. For example, using landscape position in the methodology (uplands, sideslope, floodplains) to assess groundwater recharge essentially captures several discrete DRASTIC variables, including depth to water table, net recharge, soil media, and topography. In the specific Iowa example, water tables are deeper in uplands compared to floodplains (Schilling 2009), soil media in uplands is dominated by silty loess or silty clay loam till compared to sandy soils in floodplains (Tassier-Surine et al. 2013), and net recharge varies with topography and land cover (Schilling et al. 2018). Hence, in this case study, simply classifying landscape position across the county incorporates many of the variables included in the DRASTIC methodology.
Further, the focus on aquifers directs the vulnerability assessment to groundwater supplies being used by people and not to the water table that is often unusable. In the glaciated US Midwest, the water table often resides in fine-textured sediments with low K and little water-yielding potential (Rodvang and Simpkins 2001). Hence, water supply systems and private wells often tap alluvial aquifers along rivers or upper bedrock aquifers in upland areas and protection of these vulnerable areas is paramount for communities to ensure a safe drinking water supply. In Black Hawk County, the distribution of capture zones and their relation to vulnerable floodplains or shallow bedrock areas can guide city and regional planners to protect those areas where water supply aquifers are at greater risk.
In rural areas, risks to aquifer contamination are primarily due to nonpoint source contamination from intense agricultural production. Approximately 71% of the county is overlain by annual crops of corn and soybeans (Fig. 5) and shallow groundwater throughout the county and state is threatened by high nitrate–nitrogen concentrations. For example, in one recent study, 12% of 55,000 tested private wells in Iowa had groundwater nitrate above the drinking water standard of 10 mg/l (EWG 2019). In northern Cedar Falls in Black Hawk County, Schaap (1999) observed high nitrate concentrations in shallow bedrock groundwater and these areas were identified as having the greatest aquifer vulnerability in the model (Fig. 8). In urban areas, groundwater contaminant risks are mainly from volatile organic compounds derived from poor handling of chemicals and fuels (e.g., Lapworth et al. 2012; Yu et al. 2015). Thus, overlaying potential nonpoint and point source risks on the capture zone assessment identifies areas where contaminants may be present that could impact aquifer systems.
In many areas, the presence of unweathered and unoxidized glacial till provides the dominant protection of deeper aquifers from nonpoint source contamination. With groundwater flow rates ranging from 1 m per 1000 years (Hendry 1988; Simpkins and Bradbury 1992; Remenda et al. 1996; Parker et al. 2004) to < 1 m per 10,000 years (Hendry and Wassenaar 1999; Shaw and Hendry 1998), unweathered glacial till aquitards are viewed as barriers to vertical contaminant migration. In Linn County, Iowa, Schilling and Tassier-Surine (2006) found a close spatial relationship between vulnerable bedrock aquifer regions and the absence of unoxidized pre-Illinoian till. Nitrate concentrations in excess of drinking water standards in private wells were found to be clustered in regions with little or no unoxidized till cover. In Black Hawk County, groundwater travel times to the first available aquifer in regions underlain by unoxidized pre-Illinoian till exceeded 1000 years in some areas (Fig. 4), offering protection to aquifers from contamination sourced to land surface activities. Contamination of bedrock aquifers capped by unoxidized till would be mainly due to poor well construction or regional transport of pollutants entering the aquifer at vulnerable locations including karst areas (Burkart and Stoner 2002; Kolpin et al. 1997; Schilling et al 2019).
Nitrate pollution patterns have been used to validate groundwater vulnerability assessments (Holman et al. 2005; Ghiglieri et al. 2009; Moratalla et al. 2011; Guler et al. 2013). In eastern Iowa, Schilling and Tassier-Surine (2006) showed that groundwater nitrate concentrations in private wells were highly clustered in regions with little or no unoxidized till cover. In this study, Black Hawk County wells located in vulnerable areas (scores > 20) were found to be highly correlated with groundwater nitrate concentrations greater than 3 mg/l (Fig. 10). However, it should be noted that the relations between well nitrate concentration and aquifer vulnerability scores were not perfectly correlated. First, the accuracy and scale of geologic mapping is considerably coarser than the mapping of land use and well locations. Holman et al. (2005) noted that mismatches in groundwater vulnerability can arise due to variable mapping scales and input data, and in this case, geologic map layers were interpreted across the county based on available borehole data. Resolving fine-scale spatial variability in aquifer and aquitard layers would improve spatial correlation between vulnerability scores and nitrate concentration. Second, while well locations are very good for municipal wells, private wells can be poorly located on occasion. Given that the vulnerability scoring was done at a 30 m resolution, incorrect private well locations may have a different vulnerability score than the true location of the well. Last, the vulnerability mapping does not take into account groundwater flows, travel times, and other geologic and hydrological processes that influence nitrate concentrations (Swartz et al. 2003; Holman et al. 2005; Schilling et al. 2021). Process-based modeling that would solve governing equations for unsaturated–saturated flow and contaminant fate and transport pathways would be needed to more accurately predict aquifer vulnerability at finer scales (Gogu and Dessargues 2000; Machiwal et al. 2018).
The adaptations made to the AVI methodology to account for local and regional conditions and adding new layers of assessment may provide motivation for other researchers to modify and adjust other index-based groundwater vulnerability schemes to better account for localized patterns and site-specific hydrologic controls. Off the shelf index schemes, such as DRASTIC or GOD are widely described and supported in the literature, but depending on how these generalized approaches are used, they may fail to adequately characterize vulnerability and pollution risks unique to certain areas. In this methodology, a general concept within AVI was adapted and new features and risks unique to Iowa and Black Hawk County were added. In particular, the availability of capture zone mapping and point source risks from the Iowa DNR Source Water Protection Program offered a convenient mechanism to incorporate these features into the aquifer vulnerability index. This type of data is likely available in other states, regions and countries to varying degrees.
In adapting index schemes to local settings, weighting individual factors or scoring accumulated vulnerability will be user dependent. For example, individual user choices are available to weight factors within the DRASTIC method (Aller and Thornhill 1987) and this allows users to adapt the model to local conditions. Pachero et al. (2015) observed that different weighting schemes in the DRASTIC model may produce different results. In this approach, some factors were scored on a presence or absence basis (cell within a capture zone, contains a point source, etc.) and provided a sliding scale for others (landscape position, travel time). The choice to weight some factors based on binary choices reduces uncertainty and potential variability in mapping results. Different model users should obtain the same results for groundwater vulnerability and use for the same model area since they are based on existing datasets available to the public. On the other hand, our choice to weight time of travel more than other factors was sourced to the dominant control of unoxidized till reported in previous work (Schilling and Tassier-Surine 2006) and known hydrogeologic conditions in the glaciated Midwest (Rodvang and Simpkins 2001). User decisions to estimate time of travel using different methods or with a different weighting scale may produce some variability in factor-specific or aquifer vulnerability mapping results. However, consistent with the AVI approach (Van Stempvoort et al. 1993), the key aspect of the methodology is to account for the vertical groundwater flow through low-permeable aquitards to the aquifer. Having hydrogeologic data available to estimate groundwater travel times through aquitards would reduce potential uncertainty and variability in this enhanced AVI method.
In a generalized context, the enhanced AVI methodology, which includes estimating groundwater recharge, quantifying vertical groundwater travel times, identifying potential point and nonpoint sources, and assessing groundwater use, can be easily adapted to assess aquifer vulnerability in other regions. However, the specific approach used to characterize and score the four main metrics would need to be modified to account for local and regional conditions. For example, in dry regions, groundwater recharge may need to be mapped using approaches other than simple landscape position by extrapolating from lysimeter and tracer measurements and land cover patterns (Gee and Hillel 1988; Scanlon et al. 2006). In many karst areas, groundwater recharge has been estimated by combining dye tracing studies with hydrologic models (e.g., Scanlon et al. 2003; Goldscheider and Drew 2014; Oelmann et al. 2015). Similarly, quantifying vertical groundwater travel times will require detailed understanding of heterogeneity across geographical areas of interest (Alley et al. 2002; Perrone and Jasechko 2019). Hydrogeological mapping will be critical for delineating aquifer vulnerability at local and regional scales (Thomsen et al. 2004; Diaz-Alcaide and Martinez-Santos 2019). Once aquifer depths are delineated, flow through overlying aquitard units will dominate vertical travel times (Cook and Bohlke 2000; Schilling and Tassier-Surine 2006; McMahon and Chapelle 2008). On the other hand, less variation may be associated with identification and mapping of point and nonpoint sources and groundwater use since these factors were obtained from online databases of land use/land cover patterns (nonpoint source) and source water protection programs (USEPA 2025). Overall, for users adapting the method to local or regional use, it will be important to quantify suitable metrics that capture hydrogeologic conditions and source water patterns consistent with user goals and objectives.
Conclusions
Protection of valuable groundwater supplies is needed to meet future societal demands and toward this end, groundwater vulnerability assessments are being increasingly used to guide land use and resource planning in groundwater dependent regions. In this study aquifer vulnerability in Black Hawk County, Iowa, was assessed using an index-based approach conceptually aligned with traditional AVI methodology (Van Stempvoort et al. 1993). Incorporating elements of groundwater recharge, travel time to the uppermost used aquifer, pollution risk and groundwater use, the enhanced AVI method revealed spatial variability in aquifer vulnerability at local and regional scales within the county. Increased aquifer vulnerability was evident within the Cedar River floodplain and other minor floodplain areas where there is greater recharge, faster vertical groundwater travel times, the presence of point and nonpoint sources risks and increased water use. In upland regions underlain by unoxidized till, bedrock aquifers are largely protected from groundwater pollution risks, whereas in areas where the bedrock surface is relatively shallow, the protection of unoxidized till is missing and aquifer vulnerability to contamination is higher. Groundwater use reflected by incorporating 10-year capture zones into the index model identified zones where water supply aquifers are at greater risk from point and nonpoint source contamination. The vulnerability map scores greater than 20 correlated well with groundwater nitrate concentrations greater than 3 mg/l measured in 372 public and private wells in the county (71.5%). Overall, the methodology adapts and greatly expands on the AVI approach and can be adopted in other regions as applicable. However, other researchers and users are encouraged to invest in efforts to modify and adjust other index-based groundwater vulnerability schemes to better account for localized patterns and location-specific use and risks.
Acknowledgements
We thank three anonymous reviewers and Associate Editor for their constructive comments that helped to improve the manuscript.
Declarations
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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