Scientific Investigations Report 2008-5050
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008-5050
Prepared in cooperation with the
National Water-Quality Assessment Program
Transport of Anthropogenic and Natural Contaminants (TANC) to Public-Supply Wells
By Matthew K. Landon, Brian R. Clark, Peter B. McMahon, Virginia L. McGuire, and Michael J. Turco
In 2001, the U.S. Geological Survey, as part of the National Water Quality Assessment (NAWQA) Program, initiated a topical study of Transport of Anthropogenic and Natural Contaminants (TANC) to PSW (public-supply wells). Local-scale and regional-scale TANC study areas were delineated within selected NAWQA study units for intensive study of processes effecting transport of contaminants to PSWs. This report describes results from a local-scale TANC study area at York, Nebraska, within the High Plains aquifer, including the hydrogeology and geochemistry of a 108-square-kilometer study area that contains the zone of contribution to a PSW selected for study (study PSW), and describes factors controlling the transport of selected anthropogenic and natural contaminants to PSWs.
Within the local-scale TANC study area, the High Plains aquifer is approximately 75 m (meter) thick, and includes an unconfined aquifer, an upper confining unit, an upper confined aquifer, and a lower confining unit with lower confined sand lenses (units below the upper confining unit are referred to as confined aquifers) in unconsolidated alluvial and glacial deposits overlain by loess and underlain by Cretaceous shale. From northwest to southeast, land use in the local-scale TANC study area changes from predominantly irrigated agricultural land to residential and commercial land in the small community of York (population approximately 8,100).
For the purposes of comparing water chemistry, wells were classified by degree of aquifer confinement (unconfined and confined), depth in the unconfined aquifer (shallow and deep), land use (urban and agricultural), and extent of mixing in wells in the confined aquifer with water from the unconfined aquifer (mixed and unmixed). Oxygen (δ18O) and hydrogen (δD) stable isotopic values indicated a clear isotopic contrast between shallow wells in the unconfined aquifer (hereinafter, unconfined shallow wells) and most monitoring wells in the confined aquifers (hereinafter, confined unmixed wells). δ18O and δD values for a minority of wells in the confined aquifers were intermediate between those for the unconfined shallow wells and those for the confined unmixed wells. These intermediate values were consistent with mixing of water from unconfined and confined aquifers (hereinafter, confined mixed wells). Oxidation-reduction conditions were primarily oxic in the unconfined aquifer and variably reducing in the confined aquifers.
Trace amounts of volatile organic compounds (VOC), particularly tetrachloroethylene (PCE) and trichloroethylene (TCE), were widely detected in unconfined shallow urban wells and indicated the presence of young urban recharge waters in most confined mixed wells. The presence of degradation products of agricultural pesticides (acetochlor and alachlor) in some confined mixed wells suggests that some fraction of the water in these wells also was the result of recharge in agricultural areas. In the unconfined aquifer, age-tracer data (chlorofluorocarbon and sulfur hexafluoride data, and tritium to helium-3 ratios) fit a piston-flow model, with apparent recharge ages ranging from 7 to 48 years and generally increasing with depth. Age-tracer data for the confined aquifers were consistent with mixing of “old” water, not containing modern tracers recharged in the last 60 years, and exponentially-mixed “young” water with modern tracers. Confined unmixed wells contained less than (<) 3 percent (%) young water mixed with a much larger fraction greater than or equal to (≥) 97% of old water. Confined mixed wells contained >30% young water and mean ages ranged from 12 to 14 years. Median concentrations of nitrate (as nitrogen, hereinafter, nitrate-N) were 17.3 and 16.0 mg/L (milligram per liter) in unconfined shallow urban and agricultural wells, respectively, indicating a range of likely nitrate sources. Septic systems are most numerous near the edge of the urban area and appear to be a major anthropogenic source of solutes—including nitrate-N, orthophosphate, chloride, sulfate, calcium, potassium, and boron—to unconfined shallow urban wells.
In oxic unconfined shallow urban and agricultural wells with uranium concentrations of 5 to 40 µg/L (microgram per liter), the predominant source of uranium was probably desorption or dissolution from sediment under oxic conditions and the formation of soluble complexes of uranium with calcium and bicarbonate. Although septic systems are not likely to be a source of uranium, concentrations of uranium in oxic unconfined shallow urban wells were greatest where tracers of septic-system effects occurred, perhaps because of greater calcium concentrations than in wells not affected by septic systems. The highest uranium concentrations (44 to 184 µg/L) in the study area were where water from the unconfined aquifer, inferred to have leaked downward in multi-layer wells that penetrate the upper confining unit, mixed with iron-reducing water in the upper confined aquifer. The geochemical mechanism producing these high uranium concentrations is not known but may involve dissolution of iron oxides transported with colloids from the unconfined aquifer by well-bore leakage under iron-reducing conditions in the upper confined aquifer, releasing uranium. Relatively uniform arsenic concentrations of 2 to 9 µg/L coupled with changes in arsenic speciation with increasing depth suggest that multiple processes influence arsenic concentrations, including competitive desorption of arsenate from sediments in the unconfined aquifer in the presence of orthophosphate derived from septic-system effluent, long-term reductive dissolution and oxidation-reduction reactions in the upper confined aquifer and lower confined lenses, and mixing of waters from the unconfined and confined aquifers.Samples collected from the surface discharge of the study PSW, which integrated water from the entire screened interval in the upper confined aquifer, contained concentrations of PCE, TCE, and uranium that were below drinking-water standards but of concern as indicators of low-level contamination. However, these contaminants were not detected in monitoring wells <30 m from the PSW that were screened in the same upper confined aquifer. Depth-dependent samples were collected from the PSW under typical pumping conditions at five depths in the 18-m long screen; the depths were selected on the basis of flow profiling using the tracer pulse method. The samples from the bottom half of the screen had δ18O and δD values and concentrations of PCE, TCE, major ions, excess nitrogen gas, and uranium that were consistent with those in water derived from the shallow unconfined aquifer in the urban area mixed with water from the upper confined aquifer. The presence of the water from the unconfined aquifer only at the bottom of the study PSW screen implies that well-bore leakage in the PSW itself was not the pathway for vertical movement. Similar mixtures of unconfined and confined aquifer water signatures were detected in a few monitoring wells screened in the confined aquifers. This nonuniform distribution of mixed waters implies that there were preferential flow paths permitting water and contaminants from the unconfined aquifer to move through the upper and lower confining units. The primary pathway was probably downward leakage of water through well bores or annular spaces of irrigation, commercial, or older supply wells that penetrated the upper and lower confining units.
In the PSW studied, concentrations of constituents of concern were primarily limited by dilution with “old” water from the upper confined aquifer (all constituents, particularly uranium), and chemical transformation (denitrification of nitrate, reductive dechlorination of PCE and TCE). Historical data indicate that nitrate concentrations in some PSWs in the regional-scale TANC study area screened in both the upper confined aquifer and lower confined lenses were sometimes higher than drinking-water standards, indicating that short-circuit pathways and pumping stress can overcome dilution and reaction processes under some circumstances. It is likely that concentrations of nitrate and other constituents moving with water from the unconfined aquifer to PSWs were influenced by pumping stress and the number and distance of well-bore leakage points from the PSWs.
Processes Affecting Transport of Anthropogenic and Natural Contaminants to Public-Supply Wells
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Send questions or comments about this report to the author, Matthew Landon, (619) 225-6109
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