Abstract
The types and distribution of Coastal Plain sediments in the Patuxent River Basin
may contribute to relatively low concentrations of nitrate (typically less than 1 milligram
per liter) in stream base flow because of the chemical reduction of dissolved nitrate
(denitrification) in ground water. Water chemistry data from synoptic stream base-flow
surveys in the Patuxent River Basin show higher dissolved nitrate concentrations in the
Piedmont than in the Coastal Plain section of the watershed. Stream base flow reflects
closely the chemistry of ground water discharging from the surficial (unconfined) aquifer to
the stream. Because land use in the sampled subbasins is virtually the same in each section,
differences in the physical and geochemical characteristics of the surficial aquifer may explain
the observed differences in water chemistry. One possible cause of lower nitrate concentrations
in the Coastal Plain is denitrification within marine sediments that contain chemically reduced
compounds. During denitrification, the oxygen atoms on the nitrate (N03-) molecule
are transferred to a reduced compound and N gas is produced. Organic carbon and ferrous
iron (Fe2+), derived from the dissolution of minerals such as pyrite (FeS2) and glauconite
(an iron aluminosilicate clay), can act as reducing substrates; these reduced chemical species
are common in the marine and estuarine deposits in Southern Maryland. The spatial distribution
of geologic units and their lithology (sediment type) has been used to create a map of the potential
for denitrification of ground water in the surficial aquifer of the Coastal Plain in Southern Maryland.
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First posted February 7, 2014
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