Chapter 13 - Transformations and Movement of Nitrate in Aquifer Systems

Abstract

Nitrate in ground water is subjected to a variety of physical, chemical, and biological processes which lead to changes in concentration or mass of nitrate solution. This paper reviews the processes leading to changes in nitrate concentration in ground water and discusses case studies which have examined the effects of these processes on different aquifers. Physical processes such as dilution change nitrate concentrations, but generally do not change the mass of nitrate in the aquifer. Biological and chemical denitrification are important in many aquifers for removing nitrate. Biological denitrification rates are controlled by redox conditions, available carbon and denitrifier populations. Chemical denitrification rates are controlled by redox conditions and available reduced ion species such as ferrous iron. Denitrification will lead to increases in dissolved dinitrogen gas or nitrous oxide in ground water. Dissimilatory nitrate reduction and immobilization are also of potential important in reducing the mass of nitrate. Once nitrate has entered ground water, heterogeneities within the aquifer can decrease either concentration or mass. Recharge by water with lower nitrate concentration causes dilution. Movement into confined aquifers can lead to chemical evolution of the ground water towards oxygen depletion and more reduced conditions. Movement into less permeable areas or carbon-rich portions of the aquifer can promote nitrate reduction through denitrification. Case studies of regional aquifer systems have demonstrated that nitrate disappearance occurs in contaminated aquifers although numerous interpretations of removal mechanisms are possible. Most studies used indirect measurements to indicate that biological denitrification does or can reduce nitrate levels in ground water. Studies of a contaminated aquifer on Cape Cod, MA, USA demonstrated high rates of biological denitrification. The restoration of nitrate contaminated aquifers has not been attempted by may be accomplished under certain conditions by enhancing biological or chemical denitrification rates. Increasing dissolved organic carbon and other nutrients in a contaminated aquifer would probably increase denitrification due both to the increased carbon supply and more anaerobic conditions.