Dual-tracer, non-equilibrium mixing cell modelling and uncertainty analysis for unsaturated bromide and chloride transport.

A model is presented for simulating one-dimensional advective dispersive solute transport in the vadose zone. The finite-volume, mixing-cell model uses drainage flux intervals as the index variable, which are calculated by a soil water balance model. The modelling approach considers solute transport from two different regions as well as a slow and a fast transport domain in each region as parallel transport processes. The model is applied to breakthrough curves of Cl(-) and Br(-) measured at different locations and different depths in the volcanic vadose zone of the Tutaeuaua subcatchment of Lake Taupo, New Zealand, following a dual tracer application. Estimates of transport parameter and model predictive uncertainty were derived using the differential evolution adaptive metropolis, DREAM(ZS) adaptive Markov chain Monte Carlo algorithm, a formal Bayesian likelihood function, observed leachate volumes, and Cl(-) breakthrough curves. The model was subsequently evaluated using Br(-) breakthrough curves from the dual tracer experiment and a previously conducted Br(-) tracer-only experiment. Uncertainty bounds derived by this MCMC method simultaneously capture the observed Br(-) and Cl(-) breakthrough curves and corresponding drainage volumes. Results suggest that the slow transport domain properties are relatively similar for different locations in the vadose zone and that the variability in contaminant fluxes is predominantly driven by structural variability of the vadose zone causing lateral flow.

Maximum rates of nitrate removal in a denitrification wall.

Denitrification walls are constructed by mixing a carbon source such as sawdust into soils through which ground water passes. These systems can reduce nitrate inputs to receiving waters by enhancing denitrification. Maximum rates of nitrate removal by denitrification need to be determined for design purposes. To determine maximum rates of nitrate removal we added excess nitrate (50 mg N L(-1)) to a trench up-gradient of a denitrification wall during a 9-d trial. Bromide (100 g L(-1)) was also added as a conservative tracer. Movement of nitrate and bromide was measured from shallow wells and soil samples were removed for measurements of denitrification, carbon availability, nitrate, and other microbial parameters. Rates of nitrate removal, determined from the ratio of NO3-N to Br and ground water flow, averaged 1.4 g N m(-3) of wall d(-1) and were markedly greater than denitrification rates determined using the acetylene block technique (average: 0.11 g N m(-3) of wall d(-1)). These nitrate removal rates were generally lower than reported in other denitrification walls. Denitrification rates increased when nitrate was added to the laboratory incubations, indicating that despite large nitrate inputs in the field, denitrification remained limited by nitrate. This limitation was partially attributed to nitrate predominantly moving through zones of greater hydraulic conductivity or in the mobile fraction of the ground water and slow diffusion to the immobile fraction where denitrifiers were active.

Hydraulic constraints on the performance of a groundwater denitrification wall for nitrate removal from shallow groundwater.

Denitrification walls are a practical approach for decreasing non-point source pollution of surface waters. They are constructed by digging a trench perpendicular to groundwater flow and mixing the aquifer material with organic matter, such as sawdust, which acts as a carbon source to stimulate denitrification. For efficient functioning, walls need to be permeable to groundwater flow. We examined the functioning of a denitrification wall constructed in an aquifer consisting of coarse sands. Wells were monitored for changes in nitrate concentration as groundwater passed through the wall and soil samples were taken to measure microbial parameters inside the wall. Nitrate concentrations upstream of the wall ranged from 21 to 39 g N m(-3), in the wall from 0 to 2 g N m(-3) and downstream from 19 to 44 g N m(-3). An initial groundwater flow investigation using a salt tracer dilution technique showed that the flow through the wall was less than 4% of the flow occurring in the aquifer. Natural gradient tracer tests using bromide and Rhodamine-WT confirmed groundwater bypass under the wall. Hydraulic conductivity of 0.48 m day(-1) was measured inside the wall, whereas the surrounding aquifer had a hydraulic conductivity of 65.4 m day(-1). This indicated that during construction of the wall, hydraulic conductivity of the aquifer had been greatly reduced, so that most of the groundwater flowed under rather than through the wall. Denitrification rates measured in the center of the wall ranged from 0.020 to 0.13 g N m(-3) day(-1), which did not account for the rates of nitrate removal (0.16-0.29 g N m(-3) day(-1)) calculated from monitoring of groundwater nitrate concentrations. This suggested that the rate of denitrification was greater at the upstream face of the wall than in its center where it was limited by low nitrate concentrations. While denitrification walls can be an inexpensive tool for removing nitrate from groundwater, they may not be suitable in aquifers with coarse textured subsoils where simple inexpensive construction techniques result in major decreases in hydraulic conductivity.