Meter-scale lateral heterogeneity of nitrate concentrations of groundwater in aquitards

Abstract

The long-term protection of the world's groundwater resources partially depends on the ability of aquitards to retard the movement of natural and anthropogenic solutes, and hence, it is important to understand the processes controlling the distribution, transport, and attenuation of dissolved materials. Moreover, the consequences of increasing amounts of anthropogenic N circulating in groundwater is of concern, and the attenuation and transport processes of NO₃^- present in the groundwater in aquitards must be examined. We have investigated the lateral distributions of NO₃^- concentrations in groundwater in an aquitard (4 m × 6 m area) at Tsukuba upland at a few meter-scales and attempted to identify the possible mechanisms for their heterogeneity; this, to our knowledge, is the first such study. High-resolution measurements were obtained using piezometers, and they revealed complex lateral NO₃^- heterogeneity. Large variations in the NO₃^- concentrations were found between groundwater samples from piezometers that were horizontally separated by a distance of as little as 2 m. Although the mechanism of lateral NO₃^- heterogeneity in the aquitard remained unclear, we believe that the occurrences of biochemical reactions such as denitrification and other factors such as the spatial variability of NO₃^- transport play a role in it. This first study of the meter-scale lateral heterogeneity of NO₃^- concentrations of groundwater in aquitards provides new insights into the mechanisms of natural attenuation of NO₃^- in aquitards; further, our study also discusses difficulties in some of the upscaling approaches of the NO₃^- natural attenuation zone in groundwater based on the data obtained from a small number of piezometers.