2011 Volume 41 Issue 4 Pages 113-129
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.