Multicomponent simulation of wastewater-derived nitrogen and carbon in shallow unconfined aquifiers

Abstract

One of the most common methods to dispose of domestic wastewater involves the release of septic effluent from drains located in the unsaturated zone. Nitrogen from such systems is currently of concern because of nitrate contamination of drinking water supplies and eutrophication of coastal waters. The objectives of this study were to develop and apply a mechanistic flow and reactive transport model which couples the most relevant physical, geochemical and biochemical processes involved in wastewater plume evolution in sandy aquifers. This is the first application of multicomponent reactive transport modelling to wastewater plume evolution in shallow groundwater. The work focuses on nitrogen and carbon species in wastewater because of the environmental relevance of nitrogen and the important interactions between the nitrogen and carbon chemistry systems.

The numerical model solves for variably-saturated flow and reactive transport of multiple species. Individual drains are presented in the model by using a new method based on discretization of a one-dimensional open channel flow equation; the numerical contributions to the global system of equations from the one-dimensional equation are added to the three-dimensional, porous medium contributions. The reactive transport equations are solved using the Strang splitting method which is shown to be accurate for Monod and first- and second-order kinetic reactions, and two to four times more efficient than sequential iterative splitting. The reaction system is formulated as a fully-kinetic chemistry problem which allows for the use of several special-purpose ordinary differential equation solvers. For reaction systems containing both fast and slow kinetic reactions, such as the combined nitrogen-carbon system, it is found that a specialized stiff explicit solver fails to obtain a solution. An implicit solver is more robust and its computational [performance is improved by scaling of the fastest reaction rates.

The model results were compared to geochemical data obtained from a well-studied wastewater plume in a sandy aquifer near Cambridge, Ontario. It is shown that the oxidation of ammonium and dissolved organic carbon (DOC) goes to completion in the 1.5 m distance between the drain field and the water table, and that only a minor pH reduction occurs. The overall behaviour of the reactive species in the model simulations agrees well with the geochemical data obtained below the drain field and it is concluded that the major physical and biochemical processes have been correctly captured in the current model. The model is then used to examine the impact of several key physical and chemical factors on the evolution of the wastewater plume. It is shown that depth to the water table and wastewater chemistry can have important repercussions on groundwater chemistry beneath septic drain fields. Interestingly, it is demonstrated that low-pH plumes may not always develop in noncalcareous aquifers. It is concluded that the model developed here is a useful tool to assess the impacts of onsite wastewater release into shallow aquifers.

As a final contribution the performance of an alternative drain field design is investigated. It is shown that a fine-grained layer, supplemented with labile organic carbon in the form of wood wastes, located beneath the drains is an effective means to create denitrifying conditions which eliminate nitrogen loading to shallow groundwater. It is also shown that in noncalcareous aquifers the denitrification reaction provides sufficient buffering capacity to maintain near neutral pH conditions beneath and down gradient of the drain field. Leaching of excess DOC from the denitrification layer is problematic and causes an anaerobic plume to develop in situations where the water table is less than five to six metres below ground surface: this anaerobic plume may lead to other down gradient changes in groundwater quality. A drain field and denitrification layer of smaller dimensions is shown to be just as effective for reducing nitrate, but has the benefit of reducing the excess DOC leached from the layer. This configuration will minimize the impact of wastewater disposal in areas where the water table is as shallow as 3.5 m.