Introduction
Mine waters constitute a major pollution source for the aquatic environment. Mining effluents may affect the quality of surface and groundwater as well as sediments over distances of tens of kilometres from their discharge points (Aleksander-Kwaterczak and Helios-Rybicka
2009; Liu et al.
2011). They interact with natural river water, changing the content of metals, macroions, suspended matter, oxygen, pH or temperature, and through the deposition of fine sediment, they increase the heavy metal content in a river bed and on a floodplain (Byrne et al.
2012; Ciszewski et al.
2012). Moreover, alluvial rivers may exchange waters with floodplains, changing their groundwater quality (Naiman et al.
2005).
Depending on the nature of the hydraulic gradients, groundwater flow direction within the floodplain alluvium can be dominated by components parallel or perpendicular to the channel (Larkin and Sharp
1992). The bidirectional exchange of water between groundwater and river flow takes place during both flood flows and low flows (Naiman et al.
2005; Vivoni et al.
2006). Groundwater flow directions in the groundwater/river water exchange zone, called the hyporheic zone, are controlled by channel geometry under low and intermediate flow conditions with losing, gaining and flow-through or parallel flow, and by floodplain morphology during floods (Woessner
2000). In general, most river reaches are groundwater-fed, where discharge increases by gaining water from a catchment, while many lowland, low-gradient meandering rivers lose water to the floodplain (Krause et al.
2007; Cardenas
2009). These reaches can play an important role in the transformation of pollutants transported with river water, depending on the scale of river/groundwater interactions (Gandy et al.
2007).
Interactions between groundwater and surface water bodies in river valleys are complex and depend on many factors including topography, geology, climate and the position of the surface water body relative to the groundwater flow system (Winter et al.
1998; Sophocleous
2002). The interface of groundwater and surface water along the river valley is characterised by large gradients of nutrients and trace metal concentrations, and contrasting environmental conditions (Boulton et al.
1998). Contaminants are transformed in reversible reactions such as ion exchange, adsorption or precipitation/dissolution, and irreversible reactions such as biodegradation (Lamers et al.
2006). The transformation rate is controlled primarily through the content of clay minerals, organic matter and oxides/hydroxides with the highest sorption and exchange capacity (Appelo and Postma
2005).
There are numerous examples of channel and overbank sediment contamination through metal mining, with sequences of heavy metals in overbank sediments reflecting mining history, reported from almost all regions of the world (Owens et al.
1999; Hudson-Edwards
2003; Ciszewski et al.
2012; Lecce and Pavlowsky
2014). However, while the role of sediments in contaminant transfer from the river is well known, groundwater dispersal of contaminants within a floodplain is poorly recognised. In the presented studies, it is approached through investigations of metal distribution in alluvial sediments and groundwaters of the Przemsza River floodplain, in relation to groundwater contamination at variable distances from the mine-contaminated Przemsza River in southern Poland.
The catchment of this river has for centuries been extensively mined for coal and non-ferrous metal ores. The upper course of this river, called Biala Przemsza, still receives a large proportion of waters from the lead and zinc ore mines, averaging 50 % of the river discharge. This diminishes flow amplitude, reducing the probability of overbank flows to extreme flood cases and control metal concentrations in bed sediments over the 40 km long reach (Ciszewski
2001). The lower reach of the Przemsza River also receives untreated effluents from the large industrial area of southern Poland–Upper Silesia. The river water is highly polluted with organic components of black coal, numerous plants and domestic effluents. River-borne contaminants can penetrate the alluvial floodplain as it is composed of uniform sands. Investigations focused on two reaches with contrasting groundwater exchange mechanisms: the losing reach of the Biala Przemsza River and the gaining reach of the Przemsza River. Obtained metal and macroion distribution was explained in terms of the main dispersal mechanisms.
Area of research
The Przemsza River is the left side tributary of the Vistula River, which is the main river of Poland, with over 1000 km of length. The catchment of the Przemsza River, with over 2000 km
2 of surface area, drains the eastern part of the Upper Silesia Upland and the western part of the Cracow Upland, which is dominated by carboniferous rocks. Rocks form relatively narrow cuestas; 300–400 m a.s.l., separated by wider basins situated at 250–300 a.s.l., and are filled with fluvioglacial deposits. The valley of the Przemsza River consists of a series of short and relatively steep, narrow reaches alternating with up to 1 km wide valley reaches of lower gradient filled with fluvioglacial deposits (Ciszewski
2001). In the investigated middle reach, the Biala Przemsza is a meandering, low-gradient river with low banks accompanied by a backswamp zone. Plants are typical of mid-European riparian communities, with a domination of black alder, ash, willow, as well as turf grass and reeds in locations that are lower and more distant from the channel. At valley margins, there are small oxbow lakes with natural aquatic plant communities. The radii of river meanders vary between 50 and 100 m, while the radius of the investigated compound meander is approximately 300 m. Lateral channel stability, documented by topographical maps over the period of more than the last 100 years, favoured levee accumulation in a zone 2–30 m wide and up to 1 m higher than the rest of the floodplain. The average discharge of the Biala Przemsza River equals 4 m
3/s in this reach, and half of it originates from one of the largest lead and zinc mines in Europe—the “Boleslaw” Mining and Metallurgic Plants (ZGH).
Downstream the right side tributary of the Przemsza, called Czarna Przemsza, the river receives large amounts of salts and metals from industrial wastewaters, coal mining, steel plants and untreated domestic effluents from the eastern part of Upper Silesia, which is inhabited by ca. 2 million people. The average discharge of the Przemsza River in its lower, investigated reach equals 15 m3/s. The river has a straightened channel of uniform width and stone revetments as a result of 19th/20th century channelisation. The topography of its floodplain is weakly differentiated, mostly due to artificial levelling during channelisation works. The near-bank zone is formed of low levee and associated sparse willow bushes, while remnants of the pre-regulating channel are preserved in more distant locations. In the sampling area, the left side floodplain is confined by embankments of up to a width of approximately 200 m.
Discussion
The investigated reaches were selected to represent different planform channel geometries, which control the pattern of flood sediment dispersal, floodplain topography and groundwater flow direction. Infiltration of the river water into the floodplain in the upper reach (Biala Przemsza) was anticipated based on the presence of permeable sandy sediments filling the river valley, a flat and wide valley bottom with high groundwater level, and a rapidly changing river flow direction. The beginning of the studied cross-section (BP, Fig.
1) perpendicular to the cutbank was established at the upstream part of the compound meander, where the river changes its flow from downvalley toward the valley slope direction at an angle of over 90° (Fig.
1). Considering the small channel gradient over this river reach (around 1.5 ‰), the river must lose its water laterally in the downvalley direction across the meander bend. The presence of a surface water/groundwater exchange zone within the meander bends of alluvial rivers is also predicted and considered in hydrological models (Kiel and Cardenas
2014). This phenomenon is obvious considering the very high groundwater level, almost equal to the ground surface, over the entire inner part of the meander bend, which is separated from the river by only a 20–30 m wide levee.
However, a variety of local-scale factors control the development and characteristics of the levee; its presence depends on the lateral river channel stability over medium timescales (10
2–3 years) and increases in size with drainage area and amount of suspended load (Hudson and Heitmuller
2003). Levees develop during floods along river banks and are formed due to contrasting flow velocities in the channel and over a floodplain (Wyżga and Ciszewski
2010). The levee dimensions along the Biala Przemsza River are several times lower and as much as ten times narrower than those of large lowland rivers, but their form resembles that of the largest ones with respect to their cross-sectional shape and a progressively declining lee slope. The persistence of the levee zone of the Biala Przemsza is related to long-lasting lateral channel stability, which is characteristic of low-gradient rivers with low energy flows. This stability is confirmed by the same contemporary channel course as observed at the end of the nineteenth century map, and is also suggested by the presence of dense alder carr, which stabilises channel banks. Undoubtedly, long-term channel stabilisation favours backswamp formation due to the blocking of surface drainage of that part of the floodplain by the levee. It results in the development of wetland plant communities with a relatively thick organic layer blanketing fine-grained deposits.
The investigated cross-section in the lower reach of the Przemsza represents a human-modified river with respect to its straightened course, stone bank reinforcements and weakly differentiated floodplain, as well as high water and sediment pollution. The near-bank zone was formed mainly artificially, particularly in the cross section studied, located in this part of the former channel filled with ground mass excavated during channelisation works. This zone of the cross section limits the former channel from the river, making a closed floodplain depression outside (Fig.
1). The channelisation of a meandering alluvial river such as the lower Przemsza produces a higher gradient and increases stream power and channel incision. These channel changes, leading eventually to a lowering of the groundwater table in adjacent floodplains, were also observed in other rivers of the Carpathian piedmont (Ciszewski and Czajka
2015). Also, we could reasonably assume that Przemsza River, in its lower reach, drains groundwater due to channel incision by at least 1 m, as a result of the channelisation. This leads to the conclusion that the Przemsza is a gaining river also in the cross section studied.
The assumptions regarding groundwater/surface water exchange processes in the riparian zone are confirmed by the relative changes in the chemical composition of water in the investigated cross sections. The very high content of the analysed chemical compounds in the levee zone, as compared to the rest of the cross section of the Biala Przemsza, is a characteristic feature of their distribution in the floodplain (Table
1). In particular, the drop in chloride content and conductivity between levee and backswamp indicates an intensive influx of surface water to the floodplain margin. The content of many of the investigated compounds in the river water is actually comparable to that observed immediately within the first few meters of the near-bank zone. Over the whole width of the levee, the content of calcium, magnesium and carbonates similar to or even higher than in the river can be related to the dissolution of dolomite particles originating from triassic dolomite outcrops, also observed in other drainage basins in the region (Ciszewski et al.
2012). The progressive increase of iron content from the river towards the backswamp is the other characteristic feature of the variability of water chemistry in the Biala Przemsza floodplain. High iron content in backswamp groundwater, around 1.5 mg/L on average, is well correlated with the highest iron content in surface sediments, at around 12–15 %. Swampy areas are typically abundant in iron because of widespread reducing conditions that favour the dissolution of iron compounds and precipitation when oxidisation is taking place, even producing in some locations strata of iron bog deposits (Crear et al.
1979; Lucassen et al.
2000). In the Biala Przemsza cross section, the direction of groundwater flow from the river and the increase in iron content, which could be correlated to the progressive oxygen consumption parallel to the increase in organic matter content, leads to maximum iron accumulation in the outer backswamp zone. Iron accumulation is certainly a long-term process also favoured by small water table fluctuations observed year-round, and the persistency of levee and backswamp zones. Changes in chemical composition and progressive oxygen depletion in the hyporheic zone typically induce zonation within meander bends, coupled with biochemical activity (Boano et al.
2010). However, in some losing reaches of heavily sewage-affected rivers, redox zonation can show inverted zonation from sulphate-reducing conditions, close to the river, over manganese and iron-reducing conditions to a mixed oxic/suboxic zone farthest from the channel (Sprenger and Lorenzen
2014).
The chemical composition of groundwater and its relative changes in the Przemsza cross section differs from that of the Biala Przemsza River. Moreover, the content of chlorides, sodium and potassium increases in the lower course of the river, mainly as a result of the discharge of salty waters from coal mines of the Upper Silesian mining district (Maksymiak-Lach et al.
2006). Also, total river water mineralisation rises towards the Biala Przemsza tributary from the intensively mined part of the basin. The content of the investigated metals (except of manganese) decreases, but sulphates and nitrates remain at similar levels to the upper section of the river. Despite the very high values of chlorides and mineralisation in the river water of the lower reach, they are still lower in the levee than in the levee of the upper cross section. At point P1, situated in the levee at the river bank, all the investigated values (except of zinc) are lower than both in the river and at point P2, situated farthest from the channel (Table
2). These relatively low values indicate a lack of reflux of river water towards the floodplain at average flow conditions and differ from those in the Biala Przemsza, where high values in the levee indicate a permanent influx of water from the river. The opposite distribution pattern of chemical constituents in the Przemsza reach results from the flow of groundwater towards the river and gaining conditions, which prevails at the investigated, average flows, confirming initial assumptions. Nevertheless, during prolonged periods of high river flows some loosing of water from the Przemsza River could be expected in this reach as well as gaining of water by the Biała Przemsza, during low water stages, which are rare due to constant, artificial water supply from the mine.
On many rivers, the gaining and loosing of water can be estimated by, e.g. detailed measurements of flow along the river, measurements of groundwater temperature, solute tracer or environmental tracer methods (Scanlon et al.
2002). However, the measurements of chemical constituents of both groundwater and surface water can also help to identify the extent of groundwater/surface water exchange processes in a relatively simply and inexpensive way (Kumar et al.
2009).
The measurement of chemical constituents of both groundwater and surface water can help identify the extent of groundwater/surface water exchange processes in a relatively simple and inexpensive way (Kumar et al.
2009). The spatial extent of such interactions is very evident in the investigated river reaches, with the contrasting compositions of river water and groundwater. The contrast is particularly great in the gaining Przemsza River reach as groundwater drained from the alluvial valley, over 1 km wide, is relatively weakly polluted. The higher content of many of the constituents farther from the Przemsza channel may be related to flooding, conditions favouring water stagnation in the floodplain depression, and thick sequences of polluted sediments. However, the lack of channel incision and the presence of wetland and natural geomorphic processes appear to play an important role in changes in groundwater chemistry in the Biala Przemsza cross section. The levee with the most active lateral exchange between river water and groundwater constitutes an important part of the hyporheic zone. There, water infiltrating from the river can change its properties during slow groundwater flow due to the reactive dissolution of minerals, deoxygenation, or other biochemically mediated processes controlling water chemistry transformation (Hoehn and Scholtis
2010). This suggests an important buffer capacity for the narrow strip of the floodplain (about 1/10). Farther from the channel, in a backswamp, recharge by rainfall and drainage from valley slopes most probably contribute to water chemistry.
Heavy metals are considered to be persistent contaminants which can be stored within floodplains for hundreds of years. These elements can be transferred from a channel on a floodplain during floods, both in solution as well as associated with sediments, and only in solution by bank filtration processes over a wide spectrum of flows lower than bankfull. Since the largest portion of the total metal load in rivers is transported in particulate matter, and metal concentrations in sediments are several orders of magnitude higher than in solution, overbank sediment accumulation plays a crucial role in the distribution of heavy metals in polluted river floodplains (Martin and Meybeck
1979; Miller
1997). Sediment-associated heavy metals are stored mainly in the levee zone with the highest average sediment accretion rate, which progressively decreases away from the channel (Ciszewski
2003). The same regularity is observed across the Biala Przemsza reach but peaks of metals drop, rather than progressively decline, outside the levee zone. In particular, peaks of lead, which is not a strongly mobile element, occur in the levee at the top of an approximately 20 cm thick layer, which could be attributed to the mining operation in the Biala Przemsza drainage basin since 1953. The obtained average accretion rate of the order of 3 mm/year would agree with the low levee height and reduced frequency of overbank flows due to the large and yearly constant proportion of mine water (Ciszewski
2001). This value generally agrees with accretion rates observed along the other lowland rivers (He and Walling
1996). The highest lead content occurs in the same layers as the peaks of zinc and cadmium, elements which also originate from the lead–zinc mine. This regularity did not take place in the backswamp as peaks of these metals occur at lower depths than in the levee. Considering the much lower accretion rate in the backswamp than in the levee, these must be secondary metal peaks. The post-depositional metal migration is related to the permanently high groundwater table, which fluctuates yearly in a small range within the subsurface strata (marked with a dashed line in Fig.
1). Small metal peaks, also just below the groundwater table in the levee zone, seem to confirm this conclusion, though likely the lateral component of the groundwater flow is at least just as important as the vertical one. Metal migration induced by groundwaters was also observed at the water table in other floodplains (Swennen and Van der Sluys
1998; Ciszewski et al.
2012, 2014). Between the upper and the lowest metal peaks in the levee, there is a transition strata with progressively declining metal concentrations. These changes may also be attributed to the post-depositional metal redistribution both during the periods of floodplain inundation and with rain waters. The importance of metal redistribution in the subsurface strata can be supported by high iron and manganese concentrations in a layer of greater thickness (as much as 0.5 m in the outer levee zone) than the one polluted by lead or zinc. Manganese and iron dissolution and reprecipitation are known as common mechanisms controlling the redistribution of associated metals in a hyporheic zone, as well cycling them in the water-sediment system of a river or a lake (Hudson-Edwards et al.
1998; Eggleton and Thomas
2004, Gandy et al.
2007).
While the role of groundwater in the post-depositional redistribution of heavy metals in the losing reach is important, metal redistribution in the gaining reach is less evident. Metal variability is irregular within consecutive profiles due to dumping and levelling of ground material during new bank construction across the former channel. The most recent sediment deposition could only be identified in the surface of the levee top zone, suggesting a very slow sediment accretion rate. This agrees with the presence of bank reinforcement, which precludes channel erosion and the low rate of mineral sediment transport in the Przemsza River (Lajczak
2012). The river transports mainly organic suspensions and floods usually leave only a very thin film of these materials in floodplain depressions. Overbank sediments are on average less polluted with heavy metals than in the Biala Przemsza, though to larger depths than in the levee zone. Considering that the groundwater table was reached at the bottom of the profiles, it is difficult to relate it to bottom metal peaks or to identify their regular changes with depth and distance from the river. It seems that in these profiles, groundwater plays a much less important role in the post-depositional redistribution of metals in sediments than in the losing river reach. These processes are limited by the lowering of the water table due to channel incision and overall opposite groundwater flow direction.
Conclusions
The presented investigations indicate river-to-floodplain contaminant transfer in wide and low-gradient valleys with meandering river reaches. These reaches are characterised by lateral loss of water from the channel, at least within meander bends, and by a permanently high water table favouring backswamp formation with wetland plant communities. The investigated reach of the Biala Przemsza River, which represents such conditions, is strongly contaminated with mine waters discharged from a lead–zinc mine. These conditions are conducive to the pollution of valuable wetlands over an approximately dozen kilometre-long river reach. Floodplain sediment and groundwater pollution are much higher in the levee zone than in the backswamp. Maximum zinc, cadmium and lead concentrations equal 2 %, 100 and 6000 mg/kg, respectively. The content of many chemical constituents in the groundwater as high as in the river water testifies to an intensive efflux of water from the river. In general, sediment contamination and the content of heavy metals and macroions in groundwater decrease abruptly to quite low levels over a short distance between the levee and backswamps. However, the higher content of iron in the groundwater and sediment in the backswamp proves reducible iron dissolution and precipitation at the ground surface.
Metal distribution in the top part of the levee zone in the losing reach is controlled by slow overbank accumulation of the polluted sediment. Metal distribution in the lower sediment strata of the levee and in the backswamp is affected by the permanently high groundwater table and its fluctuations. This is evidenced by the peaks of some metals shifted down relative to the surface, the tails of the upper peaks and the appearance of peaks below the average water table.
The lower course of the Przemsza River is characterised by gaining water from the floodplain due to channelisation-induced channel incision. The levee accretion over a similar period of time is negligible as compared to the natural Biala Przemsza River reach. Heavy metal concentrations vary irregularly in the profiles and are unrelated to the average groundwater table, even at a depth of over 2.5 m. The minimum groundwater pollution right at the river bank of this strongly polluted river reach was interpreted as an influx of water from the floodplain to the channel. The metal distribution in the profiles of this reach seems to be affected to a much lesser extent by fluctuations of the groundwater table than in the losing reach.