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<br /> <br />Significant denitrification has been found to take <br />place at locations where oxygen is absent or present at <br />very low concentrations and where suitable electron~donor <br />compounds, such as organic carbon, are available. Such <br />locations inciude the interface of aquifers with silt and clay <br />confining beds and along riparian zones adjacent to streams. <br />For example, in a study on the eastern shore of Maryland, <br />nitrogen isotopes and other environmental tracers were used <br />to show that the degree of denitrification that took place <br />depended on the extent of interaction between ground-water <br />and the chemically reducing sediments near or below the <br />bottom of the Aquia Formation. Two drainage basins were <br />studied: Morgan Creek and Chesterville Branch (Figure 0-2). <br />Ground-water discharging beneath both streams had similar <br />nitrate concentration when recharged. Significant denitrifica- <br />tion took piace in the Morgan Creek basin where a large <br />fraction of local ground-water flow passed through the <br />reducing sediments, which are present at shallow depths <br />(3 to 10 feet) in this area. Evidence for the denitrification <br />inciuded decreases in nitrate concentrations along the fiow <br />path to Morgan Creek and enrichment of the 15N isotope. <br />Much less denitrification took place in the Chesterville Branch <br />basin because the top of the reducing sediments are deeper <br />(10 to 20 feet) in this area and a smaller fraction of ground- <br />water flow passed through those sediments. <br /> <br />EXPLANA nON <br />NITRATE (N03l. IN MilLIGRAMS PER LITER AS N <br /> <br />~ Gee".rth"" 10 <br />5 to 10 <br />2 to 5 <br />Less than 2 <br /> <br /> <br />River in Delmarva Peninsula, Maryland. (Photograph by <br />Pixie Hamilton.) <br /> <br />Recharge <br />N03 - = 3 to 20 milligrams per titer <br />5'5 N(N03l '" 2 to 6 per mil <br /> <br /> <br />North <br /> <br />Morgan Creek <br />N03- = 2 to 3 milligrams per liter <br />615 NIN03-j = 7 to 10 per mil <br /> <br />""ate.!~\e ---- <br />,--- <br />"/,,, <br /> <br />-- '-';:: <br /> <br />Chesterville Branch <br />N03- = 9 to 10 milligrams per liter <br />815NlN03l = 4 to 5 per mil <br /> <br />South <br /> <br />Ground water <br />N03 - = 0 milligrams per liter <br />5'5N(excess N21 '" 2 to 5 per mil <br /> <br />- Base of A . <br />QUIa Formal' <br />Ion _ <br /> <br />Ground water <br />N03- = 3 to 5 milligrams per liter <br />6'5N(N03-) = 4 to 5 per mil <br /> <br />Figure 0-2. Denitrification had a greater effect on ground water discharging to Morgan Creek than to Chestervi/fe Branch in <br />Maryland because a larger fraction of the local ffow system discharging to Morgan Creek penetrated the reduced calcareous <br />sediments near or below the bottom of the Aquia Formation than the flow system associated with the Chesterville Branch. <br />(Modified from Boike, J.K., and Denver, J.M., 1995, Combined use of ground-water dating, chemical, and isotopic analyses <br />to resolve the history and fate of nitrate contamination in two agricultural watersheds. Atlantic coastal plain, Maryland: Water <br />Resources Research, v. 31, no. 9, p. 2319-2337.) <br /> <br />63 <br />