Abstract
Shallow aquifers are prone to nitrate contamination worldwide. In western Nebraska, high groundwater nitrate concentrations ([NO3−]) have resulted in the exploration of new groundwater and nitrogen management regulations in the North Platte Natural Resources District (NPNRD). A small region of NPNRD (“Dutch Flats”) was the focus of intensive groundwater sampling by the United States Geological Survey from 1995 to 1999. Nearly two decades later, notable shifts have occurred in variables related to groundwater recharge and [NO3−], including irrigation methods. The objective of this study was to evaluate how changes in these variables, in part due to regulatory changes, have impacted nitrate-contaminated groundwater in the Dutch Flats area. Groundwater samples were collected to assess changes in: (1) recharge rates; (2) biogeochemical processes; and (3) [NO3−]. Groundwater age increased in 63% of wells and estimated recharge rates were lower for 88% of wells sampled (n = 8). However, mean age and recharge rate estimated in 2016 (19.3 years; R = 0.35 m/year) did not differ significantly from mean values determined in 1998 (15.6 years; R = 0.50 m/year). δ15N-NO3− (n = 14) and dissolved oxygen data indicate no major changes in biogeochemical processes. Available long-term data suggest a downward trend in normalized [NO3−] from 1998 to 2016, and lower [NO3−] was observed in 60% of wells sampled in both years (n = 87), but median values were not significantly different. Collectively, results suggest the groundwater system is responding to environmental variables to a degree that is detectable (e.g., trends in [NO3−]), although more time and/or substantial changes may be required before it is possible to detect significantly different mean recharge.
Highlights
Elevated groundwater nitrate concentrations ([NO3 − ]) in shallow aquifers are often linked to a combination of high groundwater recharge rates and intensive agricultural land use [1,2,3,4,5,6]
Groundwater samples analyzed for 3 H/3 He, NO3 −, δ15 N-NO3 −, and δ18 O-NO3 −, among other groundwater parameters, were used to evaluate the hypothesis that changes in environmental variables since the previous study would: (1) decrease recharge rates; (2) increase biogeochemical activity; and (3) result in lower groundwater [NO3 − ]
Long well screens across the water table can increase error in groundwater age, due to mixing of a range of groundwater ages, and because fluctuations in the water table can lead to a loss of tritium-derived 3 Hetrit escaping to the atmosphere
Summary
Elevated groundwater nitrate concentrations ([NO3 − ]) in shallow aquifers are often linked to a combination of high groundwater recharge rates and intensive agricultural land use [1,2,3,4,5,6]. High [NO3 − ] have been observed in groundwater in Nebraska, especially beneath areas with sandy soils and/or sand and gravel aquifers [9,10,11,12,13]. Established in 1972, NRDs develop management plans and regulations to protect groundwater [14,15,16]. Due to the tendency of nitrate to be transported with recharge water, agricultural water management (i.e., irrigation technology and practices) and groundwater [NO3 − ]
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