WHAT, ME WORRY? NONPOINT SOURCE CONTAMINATION OF A MUNICIPAL WATER SUPPLY AT THE URBAN-AGRICULTURAL INTERFACE

The City of Ames, Iowa (pop. 50,731) relies on drinking water from a large alluvial (unconfined)/buried valley (confined) aquifer. Groundwater is pumped by 22 wells in four well fields within city limits, but wells also induce flow from streams receiving nutrient-rich runoff and tile drainage from intensive row-crop (corn-soybean) production. As part of a 5-year study of the aquifer, Cl, NO₃-N, and P concentrations were used along with groundwater age dating (³H/³He, ³H, and SF₆) and a 3-D, finite-difference model (MODFLOW) to assess the extent of nonpoint source contaminants in the aquifer and future threats to the water supply. The results provide evidence of contamination from both de-icing salt and agricultural fertilizer. Maximum groundwater age in the aquifer is 35 years, a throughput consistent with the groundwater model. Cl/Br ratios of 745.4 and 729.9 from two municipal wells suggest anthropogenic Cl contamination. Chloride concentrations from municipal wells range from 10 to 121.5 mg/L with a mean of 45 mg/L. Although concentrations of Cl in streams range between 20 and 30 mg/L, Cl and Na concentrations (meq) plot along 1:1 line and the highest concentrations occur adjacent to highways. Hence, de-icing salt is likely the main source of Cl. Concentrations in the wells have also increased from values < 20 mg/L since 1969, consistent with increased use of salt by the city during the past 40 years. In contrast, agricultural fertilizer in streams is likely the main source of nutrients to the aquifer. Denitrification removes NO₃-N in stream water upon entering the aquifer, but total dissolved P concentrations, maintained by low redox conditions in the aquifer, range from 88 to 653 µg/L, with a mean of 184 µg/L. The mean is similar to concentrations in shallow groundwater immediately adjacent to the South Skunk River, while maximum concentrations are consistent with reported stream concentrations of 600 µg/L. In summary, the hydrogeology, short residence time, and pervasive nonpoint source contaminants in the aquifer suggest that it is vulnerable to contamination from a variety of sources. Given the lack of protection for the aquifer recharge areas, declining water quality in Iowa’s rivers and the presence of emerging contaminants therein, residents of Ames should be worried about the future quality of their water supply.