Identifying sources of groundwater nitrate contamination in a large alluvial groundwater basin with highly diversified intensive agricultural production
Introduction
Elevated nitrate levels (more than 2 mg/L NO3-N) in groundwater used as drinking water have been linked to adverse health effects (Mueller and Helsel, 1996). Consumption of water containing elevated levels of nitrate can cause low blood oxygen in infants, a condition known as methemoglobinemia or “blue baby syndrome”. Methemoglobinemia was the impetus behind the United States Environmental Protection Agency (USEPA) maximum contamination level (MCL) of 10 mg/L NO3-N (Mueller and Helsel, 1996). Nitrate in drinking water has also been linked to cancer through the formation of carcinogenic N-nitroso compounds (Weyer et al., 2001), to spontaneous abortions (Centers for Disease Control and Prevention, 1996), and to non-Hodgkin's lymphoma (Ward et al., 1996).
Nitrate occurs naturally in groundwater. However, septic leakage, nitrogen fertilizers, and animal manure applied to soil can cause elevated levels of nitrate in groundwater (Owens et al., 1992). High groundwater nitrate has been positively correlated with surrounding agricultural landuse (Vowinkel and Tapper, 1995). In the San Joaquin Valley (SJV) (Fig. 1) as much as 88 kg N/ha/year may leach to groundwater in areas where fertilizers are applied (Harter, 2009). Leaching from dairy corrals, ponds, and from fields receiving manure may be as high as 872 kg/ha/year, 807 kg/ha/year and 486 kg/ha/year, respectively (van der Schans et al., 2009). Increasing trends in nitrate levels in SJV groundwater during the 1950s and 1960s and from the 1970s to 1980s correlated with an increase in fertilizer and manure use, and an increase in confined animal feeding operations (CAFOs) in the SJV over the same time period (Dubrovsky et al., 1998).
Approximately two-thirds of the SJV landscape is in agricultural production (Burow et al., 2008a). More than 250 unique crops are grown in the SJV. It is home to three-quarters of California's dairy herd. The annual gross value of agricultural production in the SJV is more than $25 billion (United States Environmental Protection Agency, 2012). Irrigation water is supplied by both surface water and groundwater, while groundwater is the almost exclusive source of drinking water in rural and embedded urban areas such as Stockton, Modesto, Fresno, Tulare, and Bakersfield (Burow et al., 1998b). Total population for the eight counties in the SJV (Fresno, San Joaquin, Kern, Stanislaus, Tulare, Merced, Kings, and Madera) in 2006 was nearly 3.9 million (California Department of Finance, 2006).
Nitrate contamination of shallow groundwater (≤ 150 m deep) in the SJV is well documented. Twenty groundwater study units, distributed throughout the nation, were compared as a part of the U.S. Geological Survey (USGS) National Water Quality Assessment Program (NAWQA). Among the twenty NAWQA study units, the SJV (also referred to as the San Joaquin–Tulare Lake Basin) had nitrate concentrations in groundwater above the national median (Dubrovsky et al., 1998). The 2006 California State Water Resources Control Board (SWRCB) Groundwater Ambient Monitoring and Assessment Program (GAMA) study of 181 domestic wells in Tulare County (including wells located in the foothills outside the SJV) found that 40% of well water samples exceeded the nitrate MCL (California State Water Resources Control Board, 2010). A similar study conducted in Merced County in 2001 on 40 domestic wells found 63% to exceed the MCL for nitrate (Harter and Romesser, 2001).
Previous studies conducted in agricultural areas overlying unconsolidated aquifers determined a significant relationship between crop type or landuse within circular well buffer zones centered on sampled wells and well water nitrate (Burow et al., 1998a, Kolpin, 1996, McLay et al., 2001). However, previous studies, typically including 50 to 100 well sites, have been limited to relatively few crop type and landuse classifications (Burow et al., 1998a, McLay et al., 2001) or overarching categories such as “irrigated agriculture” (Kolpin, 1996). Studies have also shown that nitrate in groundwater can be affected by vadose zone thickness (Burow et al., 1998b) and soil type (Burow et al., 1998a) and that nitrate in well water samples can be affected by well construction characteristics such as well depth (Burow et al., 1998b).
This study expands on previous work using a larger sample size across a wider diversity of agricultural crops and landuses. The goal of this study is to determine how various landuses affect groundwater nitrate and how other factors, such as well depth, may play a role in the amount of nitrate found in well water samples. Specifically, we consider 78 crop and landuse types (grouped into 10 categories), proximity to dairies, vadose zone thickness, soil type, and well construction characteristics.
Section snippets
Project area description
The study area is located in the San Joaquin Valley (SJV), which represents the southern portion of the Great Central Valley of California. The SJV is a structural trough up to 322 km (200 miles) long and 113 km (70 miles) wide (DWR, 2004) that is filled with up to 10 km (6 miles) of marine and continental sediments (Page, 1985) deposited by the Pacific Ocean and erosion of the surrounding mountains. Fresh groundwater is found in tertiary and quaternary alluvial sediments comprising the upper 500 to
Well depths and screen lengths
In Stan/Mer, screen length for the sampled wells has a mean of 11 m and a median of 6 m and completed well depth has a mean of 55 m and a median of 55 m. In Tul/Kings, screen length for the sampled wells has a mean of 27 m and a median of 20 m and completed well depth has a mean of 73 m and a median of 61 m (Fig. 4). In general, wells sampled in Tul/Kings have longer screened intervals and were deeper than wells sampled in Stan/Mer.
When compared using the SRC, the study wells did not have a significant
Conclusions
Overall, domestic wells in Stanislaus, Merced, and Tulare Counties (Kings County to a lesser extent) are widely affected by nitrate contamination above regulatory limits. That contamination is most strongly associated with CAFO manure lagoons and animal corrals and with forage, citrus and deciduous fruit and nut crops.
Depth to groundwater provides significant control on nitrate concentration in domestic wells with higher values mostly where the water table is shallower < 21 m (70 ft) and lower
Acknowledgments
This work was funded by the California State Water Resources Control Board Grant Agreement No. 04-184-555-0. We would like to thank the following people and groups for their help and support on this project: landowners who allowed us to sample wells on their property; Joseph Trujillo and Olin Applegate of UC Davis for collecting well samples; UC Davis Analytical Lab; Ronald Bond and Xunde Li of the UC Davis microbiology lab, Nate Roth and Jim Quinn of the UC Davis Information Center for the
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