A CHEMICAL AND ISOTOPIC COMPARISON OF TWO 1ST-ORDER AGRICULTURAL TRIBUTARIES, KENT COUNTY, MARYLAND

Abstract

Nitrate derived predominantly from agricultural fertilizers results in algal blooms and wide-spread anoxia in estuarine environments. Roughly half the nitrate delivered to the Chesapeake Bay is derived from groundwater (residence time up to 40 years). Two 1st-order tributaries in eastern Maryland were sampled on five dates. Over the study interval, average nitrate, alkalinity, and delta 13C were 207 µM, 212 µM, and -12.1, respectively, in the ditched tributary, and 106 µM, 451 µM, and -9.7, respectively, in the unaltered tributary.

Ditching of the western tributary results in discharge of less anthropogenically-altered groundwater. Nitrate, calcium, and magnesium concentrations decrease and 13C abundance of DIC becomes enriched along the reach. The unaltered tributary is stagnant in the headwaters, resulting in consumption of dissolved oxygen and denitrification. Alkalinity correspondingly increases suggesting reduced carbon as the electron source for this microbial process. Alteration of 1st-order tributaries influences the processing and delivery of nutrients.