An integrated approach to hydrogeologic investigations — A case history

Abstract

This paper presents a case history in which several investigative techniques were applied to a particular hydrogeologic problem. The study addressed the occurrence and migration of nitrate in a thin (3–7 m) phreatic sand aquifer beneath agricultural land. At an early stage in the investigation, high nitrate concentrations were found at shallow depths in the aquifer, while at greater depth in the aquifer the nitrate concentration was close to zero. The interface between the shallow groundwater containing nitrate and the deeper nitratefree water was sharp throughout the study area. Two explanations for the nitrate distribution were considered: (1) the shallow high-nitrate water originated as infiltration through the soil of the study area and the deeper nitrate-free water entered the study area from upgradient of the study area; and (2) the vertical nitrate distribution was the result of denitrification in the aquifer. Through the application of physical hydrogeologic methods of investigation (test drilling, groundwater instrumentation, hydraulic-head monitoring and hydraulic conductivity measurements), geochemical studies (major-ion analyses, pH, Eh, dissolved oxygen, methane and alkalinity), environmental isotope studies ( 18O, deuterium and tritium), numerical simulations of groundwater flow and solute transport and an analytical solution of solute transport, it was shown that denitrification is the principal cause of the nitrate distribution and the first hypothesis was invalid. The study showed that denitrification can be an important process in groundwaters on a regional scale. Without the application of an integrated investigative approach, the importance of the denitrification process would probably not have been recognized and confirmed. Although not easily anticipated, it is possible that over-interpretation of results from any one of the study methods could have produced erroneous conclusions. This paper demonstrates some of the limitations and advantages of the various methods used in the investigation.