Combined Use of Groundwater Dating, Chemical, and Isotopic Analyses to Resolve the History and Fate of Nitrate Contamination in Two Agricultural Watersheds, Atlantic Coastal Plain, Maryland

Abstract

The history and fate of groundwater nitrate (NO•-) contamination were compared in 2 small adjacent agricultural watersheds in the Atlantic coastal plain by combined use of chronologic (CC12F2, 3H), chemical (dissolved solids, gases), and isotopic (•15N, •13C, •34S) analyses of recharging groundwaters, discharging groundwaters, and surface waters. The results demonstrate the interactive effects of changing agricultural practices, groundwater residence times, and local geologic features on the transfer of NO•through local flow systems. Recharge dates of groundwaters taken in 1990-1992 from the surficial aquifer in the Chesterville Branch and Morgan Creek watersheds near Locust Grove, Maryland, ranged from pre-1940 to the late 1980's.When corrected for localized denitrification by use of dissolved gas concentrations, the dated waters provide a 40-year record of the recharge rate of NO•, which increased in both watersheds by a factor of 3-6, most rapidly in the 1970's. The increase in groundwater NO•-over time was approximately proportional to the documented increase in regional N fertilizer use, and could be accounted for by oxidation and leaching of about 20-35% of the fertilizer N. Groundwaters discharging upward beneath streams in both watersheds had measured recharge dates from pre-1940 to 1975, while chemical data for second-order reaches of the streams indicated average groundwater residence times in the order of 20+ years. At the time of the study, NO•-discharge rates were less than NO•-recharge rates for at least two reasons: (1) discharge of relatively old waters with low initial NO•-concentrations, and (2) local denitrification. In the Chesterville Branch watershed, groundwaters remained oxic throughout much of the surficial aquifer and discharged relatively unaltered to the stream, which had a relatively high NO• concentration (9-10 mg/L as N). In the Morgan Creek watershed, groundwaters were largely reduced and denitrified before discharging to the stream, which had a relatively low NO•-concentration (2-3 mg/L as N). Chemical and isotopic data indicate that quantitative denitrification occurred within buried calcareous glauconitic marine sediments that are present at relatively shallow depths beneath the Morgan Creek watershed. NO•-removal by forests, wetlands, and shallow organic-rich soils in near-stream environments was largely avoided by groundwaters that followed relatively deep flow paths before converging and discharging rapidly upward to the streams. This paper is not subject to U.S. copyright. Published in 1995 by the American Geophysical Union.Paper number 95WR01584. and although many potential causes of such imbalances are known, the essential causes in specific field situations are not always easy to resolve. Watershed NO•-gradients can have a variety of different configurations that depend locally on both transient and steady state variables, including (1) changes in the N contents of recharging groundwaters over time (related to changes in land use practices), (2) the distance, direction, a...

Probability of Nitrate Contamination of Recently Recharged Groundwaters in the Conterminous United States

“…Relations between groundwater nitrate concentration and N fertilizer or agricultural land use are well-documented (2,13,21,37,38). Relations among percent population density, percent well-drained soils, and depth to the seasonally high water table are consistent with the previous model (20).…”

Dating young groundwater with sulfur hexafluoride: Natural and anthropogenic sources of sulfur hexafluoride

“…For this reason, if the presence of excess air is not considered in the calculation of an SF6 model age, then the apparent age will be too young. The error in the age due to the presence of excess air is less than 1 year for pre-1990 waters dated with CFCs [Busenberg and Plummer, 1992] but can be significant for younger waters. Excess air can introduce a significant error when dating groundwater with SF6 because the Henry's law constant of SF6 is approximately 55 and 13 times smaller than that of CFC-11 and CFC-12, respectively.…”

Water Quality Functions of Riparian Forest Buffers in Chesapeake Bay Watersheds

“…Similar results were inferred from a study of nitrate in regional groundwater and nitrate levels in streamflow for the WDU hydrogeomorphic region (Phillips and others 1993). In related work, Bohlke and Denver (1995) concluded that a riparian forest wetland next to a stream in the WDU had little effect on nitrate movement to the stream. Hydrologic data and groundwater flow modeling showed that groundwater discharges upward directly to the streambed from the aquifer system, effectively bypassing the riparian zone (Reilly and others 1994).…”

“…The general conclusion of all these studies was that denitrification occurred in most riparian forest soils, especially in the root zone, or that conditions were favorable for denitrification. Recent work by Bohlke and Denver (1995) indicated that denitrification can also occur in aquifer sediments below the influence of the riparian root zone where proper geochemical environments exist. Isotopic analy- sis of groundwater and streamflow at this site indicated that denitrification was not significantly reducing nitrate concentrations (Bohlke and Denver 1995).…”

“…Recent work by Bohlke and Denver (1995) indicated that denitrification can also occur in aquifer sediments below the influence of the riparian root zone where proper geochemical environments exist. Isotopic analy- sis of groundwater and streamflow at this site indicated that denitrification was not significantly reducing nitrate concentrations (Bohlke and Denver 1995). Denitrification was measured in riparian forests of Little River, Georgia, in conjunction with water quality and hydrologic measurements (Hendrickson 1981).…”