Vadose Zone Nitrate-N Study: Final Report Hastings Wellhead Protection Area: City of Hastings, NE
Document Type Article
An improved understanding of the occurrence, rate of transport, and breakdown of agrichemicals in the vadose zone allows municipalities to better anticipate and predict groundwater contamination. By sampling previously collected sites, it is possible to determine if changing practices and the use of BMPs such as improvements in water and fertilizer application input have a measurable effect on nitrate-N loading to the vadose zone and the underlying groundwater. Quantifying the contaminant mass in the entire vadose zone allows for a more complete representation of stored agrichemicals. It also more effectively reveals nitrate-N concentrations in recharge water close to the groundwater table. Recharge water that is approaching or exceeding the 10 mg/L MCL for nitrate-N has implications towards water quality within the capture zones of municipal wells. Concentrations of ammonium-N should also be taken into consideration, as it also has been observed accumulating in the vadose zone and can be biologically converted to nitrate-N under certain conditions. This investigation quantified the mass of agrichemicals in Hastings’ WHPA and compared them to estimations made five years previously in a 2011 study (R. Spalding & Toavs, 2011). Land use among the sampled locations varied from urban land, pivot/gravity irrigated cropland, and non-irrigated cropland. Certain lithologic properties seemed to correlate with concentrations of agrichemicals. High nitrate-N concentrations were commonly found in sediments consisting of clay and silt loams. Overall, fluctuations of stored nitrate-N varied site by site over the five-year span. Potential nitrogen sources at these sites varied from nonpoint sources in row-cropped farmland to suspected point source releases (R. Spalding & Toavs, 2011). Producer fields increased by 2,800 lbs-N/Acre of stored nitrate-N in the top 60 ft. Sites that were converted from gravity to pivot irrigation showed a reduction of approximately 170 lbs-N/acre in the top 55 ft of the profile over a five-year time span. This reinforces the idea that irrigation management can be an effective BMP to protect groundwater quality. Overall, amount of nitrate-N stored under urban lawns decreased by 840 lbs-N/Acre. The amount of vadose zone contamination from urban locations depends on factors similar to agricultural regions, such as water input, fertilizer usage, and land use within the urban environment. Cumulative nitrate-N beneath the top 65 ft for urban irrigated lawns, pivot irrigated farmland, and gravity irrigated farmland had an average of 320, 540, and 700 total lbs-N/acre respectively. Although no significant differences between their nitrate-N were present at the different depths, trends of higher nitrate-N under cropland vadose zones were present.