SOURCE DISTRIBUTION OF NITRATE AROUND MUNICIPAL WELLS OF A SUBURBAN TOWN IN IOWA

Groundwater nitrate contamination has been a persistent concern in the drinking water of Cedar Falls, Iowa. Identifying the source of the relatively high nitrates in Cedar Falls Municipal Well #3 is the primary focus of this investigation. Although none of the municipal wells have exceeded the maximum contaminant level for nitrogen, the purpose of this project is to investigate the general drinking water quality. To identify the source of the nitrate contamination, groundwater samples from 4 municipal wells, surface water samples from 3 sites on the Cedar River, and soil from 12 sites within one mile radius’ of the municipal wells were collected. A total of 50 water samples and 60 soil samples were collected for ten weeks from May until July 2012. Onsite parameters for water samples included Dissolved Oxygen (DO), pH, Total Dissolved Solids (TDS), Conductivity, and Temperature. For soils, nitrate was extracted using 2g of soil in 50mL of distilled water that was shaken for 4 hours. Soil moisture and organic content were analyzed by the method called Loss-on-Ignition (LOI). Major ion chemistry of water was determined by ion chromatography. The results of this investigation show relatively higher nitrogen content in the soils around Well #3 (average nitrate (as NO₃) concentrations of .073 and .127 mg Nitrate/g Soil) and Well #11 (average nitrate (as NO₃) concentrations of .063 and .064 mg Nitrate/g Soil) that is derived through vertical infiltration of soil nitrate directly from the field through preferential pathways. The nitrate (as NO₃) level tested in Well #3 ranged from 34.4 ppm on 7/18/2012 to 39.5 ppm on 7/5/2012. Although nitrate coming from a far-away source through lateral migration within the aquifer cannot be ruled out at this point, predominant evidences point to local origin of nitrate. It is likely that organic nitrogen that accumulated in the soil over many years of fertilization in the past is now being slowly released into the soil through the process of nitrification into inorganic nitrate. Soil nitrate is expected to decrease through the process of denitrification because the area is now predominantly residential and there is no major source of nitrate in the soil. All other hydrologic parameters in the area are within the expected range of limits.