Investigations into the distribution of non-point source nitrate in two unconfined aquifers and the role for carbon addition in the control of nitrate concentrations in groundwater

• Main Author: Dasika, Raghava Kumar
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/6301

Shallow unconfined aquifers are prone to contamination by non-point source nitrate as a result of agricultural and other land use practises. Management of such susceptible groundwater resources requires a means of quantifying the transport and fate of the nitrate from its source to the water table and its subsequent distribution through the groundwater. A proper understanding of the dynamics associated with nitrate loading and its effect on unconfined groundwaters is currently lacking. It has been suggested by several investigators that in-situ management of groundwater nitrate may be achieved by promoting the natural denitrification capacity of microbial populations present within the aquifers. However, the feasibility of such an approach has not yet been fully evaluated. A study involving the detailed monitoring of the changing groundwater chemistry profiles beneath the water table of two unconfined aquifers, combined with a laboratory column-test investigation of enhanced denitrification during saturated flow through sand, has been performed. The groundwater monitoring was performed using a passive sampling approach that has enabled detailed multi-level profiling of the groundwater chemistry at and below the water table. Measurements were made monthly, over a period of twelve months, in order to determine the temporal variation in groundwater chemistry as related to the surficial land use and recharge patterns. This research has included the first known detailed measurement of the seasonal arrival of leached non-point source nitrate to the water table beneath agricultural lands, and has provided definitive evidence of the pulsed nature of such arrival. At the study sites, the monitoring has also shown that infiltrating recharge waters result in the rapid delivery of leached nitrate to the water table of the unconfined aquifers. Furthermore, the research findings suggest that, as a result of the development of vertical hydraulic gradients at the water table during recharge, leached nitrate fronts can be transported to a large depth below the water table within a short time period. A conceptual model has been developed to describe the observed distribution of nitrate below the water table. Using this conceptual model, it is also concluded that nitrate that arrives at the water table of unconfined aquifers with seasonal recharge waters will subsequently be transported through the aquifer in a pulsed manner in response to a corresponding seasonal fluctuation in the groundwater flow velocities. The laboratory column testing has found that ethanol may be preferred over methanol as a carbon source by the subsurface microbial population during enhanced denitrification. The findings from this testing also suggest that the effectiveness of carbon injection systems, as well as stationary reactive barriers aimed at providing denitrifying environments, may be compromised by clogging as a result of N2 gas bubble accumulations. The study results suggest that the role for carbon additions within aquifers for promoting in-situ denitrification, on an aquifer wide basis, may be limited due to the seasonally dynamic nature of the nitrate loading and subsequent distribution through unconfined aquifers, as well as uncertainty associated with the efficacy of heterotrophic denitrifying microbial populations within the aquifers.