Strontium isotope geochemistry of groundwaters and streams affected by agriculture, Locust Grove, MD

The effects of agriculture on the isotope geochemistry of Sr were investigated in two small watersheds in the Atlantic coastal plain of Maryland. Stratified shallow oxic groundwaters in both watersheds contained a retrievable record of increasing recharge rates of chemicals including NO3-, Cl, Mg, Ca and Sr that were correlated with increasing fertilizer use between about 1940 and 1990. The component of Sr associated with recent agricultural recharge was relatively radiogenic (Sr-87/Sr-86 = 0.715) and it was overwhelming with respect to Sr acquired naturally by water-rock interactions in the oxidized, non-calcareous portion of the saturated zone. Agricultural groundwaters that penetrated relatively unoxidized calcareous glauconitic sediments at depth acquired an additional component of Sr from dissolution of early Tertiary marine CaCO3 (Sr-87/Sr-86 = 0.708) while undergoing O-2 reduction and denitrification. Ground-water discharge contained mixtures of waters of various ages and redox states. Two streams draining the area are considered to have higher Sr-87/Sr-86 ratios and NO3- concentrations than they would in the absence of agriculture; however, the streams have consistently different Sr-87/Sr-86 ratios and NO3- concentrations because the average depth to calcareous reducing (denitrifying) sediments in the local groundwater flow system was different in the two watersheds. The results of this study indicate that agriculture can alter significantly the isotope geochemistry of Sr in aquifers and streams and that the effects could vary depending on the types, sources and amounts of fertilizers added, the history of fertilizer use and groundwater residence times. (C) 2000 Elsevier Science Ltd. All rights reserved.