Transport of Bromide and Other Inorganic Ions by Infiltrating Storm Water Beneath a Farmland Plot

Abstract

Abstract A hydrogeochemical study in the clay‐soil mantled portion of a karst terrane in southern Indiana demonstrated preferential flow of soil water through macropore structures that are of higher permeability than the soil matrix. Variable transport of several inorganic ions, such as Cl – , SO 4 = , Na + , Ca ++ , and Mg ++ , through the unsaturated zone was observed immediately after a major storm event in April, 1992. A KBr salt spray experiment during the storm led to recovery of bromide, a nonreactive, conservative ion, at a depth of 10 ft within 24 hours after the storm. The observed Br − transport rate (5 in/hr) is approximately six times greater than the expected rate of transport in the absence of preferential flow. Temporal changes in cation to anion ratios demonstrated that anions are more mobile due to the process of anionic exclusion which drives more anions out of the clay matrix whereas cations are attenuated by adsorption on the clay particles due to their charge differences. In the absence of storm‐water infiltration, the ion transport proceeds largely by matrix flow of soil water. The storm event also initiated considerable mixing of ground water through lateral flow within the aquifer. The ions, such as Cl – , NO 3 , Ca ++ , and Mg ++ , showed immediate dilution by storm water. Subsequently, ion concentrations in the aquifer increased to the initial level because of reequilibration with soil matrix water released from the lower part of the unsaturated zone.