Effects of episodic rainfall on a subterranean estuary

Numerical simulations were conducted to examine the effect of episodic rainfall on nearshore groundwater dynamics in a tidally influenced unconfined coastal aquifer, with a focus on both long- (yearly) and short-term (daily) behaviour of submarine groundwater discharge (SGD) and seawater intrusion (SWI). The results showed non-linear interactions among the processes driven by rainfall, tides and density gradients. Rainfall-induced infiltration increased the yearly averaged fresh groundwater discharge to the ocean but reduced the extents of the saltwater wedge and upper saline plume as well as the total rate of seawater circulation through both zones. Overall, the net effect of the interactions led to an increase of the SGD. The nearshore groundwater responded to individual rainfall events in a delayed and cumulative fashion, as evident in the variations of daily averaged SGD and salt stored in the saltwater wedge (quantifying the extent of SWI). A generalized linear model (GLM) along with a Gamma distribution function was developed to describe the delayed and prolonged effect of rainfall events on short-term groundwater behaviour. This model, validated with results of daily averaged SGD and SWI from the simulations of groundwater and solute transport using independent rainfall datasets, performed well in predicting the behaviour of the nearshore groundwater system under the combined influence of episodic rainfall, tides and density gradients. The findings and developed GLM form a basis for evaluating and predicting SGD, SWI and associated mass fluxes from unconfined coastal aquifers under natural conditions, including episodic rainfall.

Tide-induced surface water and groundwater interactions in coastal wetlands

David Andrew Barry

Intertidal wetlands such as salt marshes are complex hydrological systems characterized by strong, dynamic interactions between coastal surface water and groundwater, driven particularly by tides. We simulated such interactions with a focus on 3D, variably saturated pore water flow in a salt marsh with a two-layer soil configuration (with a low-permeability mud layer overlying a high-permeability sandy-loam layer), which is commonly found in natural marshes. Simulated intra-tidal groundwater dynamics exhibited significant flow asymmetry with non-zero mean flow velocities over the tidal period. The tidally averaged flow led to 3D pore water circulation linked strongly to the marsh topography, over a range of spatial scales: near the creek bank, around the creek meander and over long marsh sections inclined towards the main channel. Time scales associated with these circulations differed by orders of magnitude. Under the simulated conditions, the creek served as the main outlet of the pore water circulation paths, especially those with infiltration taking place in the upper marsh surface areas away from the main channel. Water infiltrating the soil in the lower marsh surface areas away from the creek tended to discharge to the main channel directly. These flow characteristics have important implications for mass and nutrient transport and transformations in the marsh soil. Since the origin of pore water in the marsh soil is largely the coastal surface water, the travel paths and times revealed by the particle tracking are key factors that determine the (modified) chemical composition of the recycling water at the circulation outlet, which in turn affects the net exchange between the marsh and coastal surface water. Our study highlights the hydrological complexity of intertidal marshes and the need for further research on interactions among marsh morphology, hydrology and ecology, which underpin the functionalities of these wetland systems.

2011

Utilization of Silicate Minerals for pH Control during In Situ Bioremediation of Chlorinated Solvents

David Andrew Barry, Alessandro Brovelli, Christof Holliger, Elsa Marina Lacroix

Background/Objectives. Chloroethenes such as tetrachloroethene (PCE) and trichloroethene (TCE) are among the most prevalent contaminants in groundwater due to their extensive use in industrial processes. In situ bioremediation (ISB) is an attractive technology for the removal of these compounds. It has been widely used for treatment of chloroethene plumes and recent studies have shown promising results for removal of dense non aqueous phase liquids (DNAPLs). However, application of source zone ISB is still a significant technical challenge. One of the main issues is groundwater acidification due to organohalide respiration and fermentation processes, which can inhibit the activity of dehalogenating micro-organisms. The main objective of this work was to develop an efficient pH control strategy for chloroethene ISB by using the acid neutralizing potential of silicate minerals. These minerals are of particular interest as their dissolution rate and their solubility is pH dependent with faster kinetics and higher solubility in the acidic range. In addition, they persist in the subsurface and are long term alkalinity sources. Their usage therefore potentially reduces the frequency of injection of buffering solution. Approach/Activities. To assess the potential of this technology, modeling and experimental approaches were combined. A geochemical model, implemented within PHREEQC, was developed to select appropriate buffer candidates and to help determine main parameters influencing mineral buffering capacity. The model included microbial degradation kinetics, mineral dissolution kinetics and chemical speciation. Anaerobic microcosm experiments were carried out to compare the buffering capacity of ten silicate minerals and to investigate interactions between minerals and dehalogenating bacteria, e.g., the potential inhibitory effect of minerals on the dehalogenating activity. Four sets of microcosms with four different consortia were conducted. Each microcosm was amended with either 5 mmol l-1 of PCE or or cis-DCE and a defined amount of mineral powder with grain sizes between 50 and 100 μm. Abiotic mineral dissolution experiments were also conducted on promising buffer candidates to give a precise estimation of keys parameters such as equilibrium and kinetics constant without the presence of bacteria. Results of these abiotic tests were used to validate and calibrate the geochemical model developed. Results/Lessons Learned. The results of model simulations confirmed that the efficiency of the system is dependent on mineral dissolution kinetic constants, equilibrium constants, temperature and reactive surface area. The geochemical model and literature parameter data were used to pre-select minerals with a buffering capacity sufficient to counterbalance acidity produced by dehalogenating bacteria at a rate of 4 mmol l-1 d-1 of chloride. From the 31 silicate minerals with published kinetic data, 10 suitable buffering candidates were identified. Results of microcosm experiments with silicate minerals and dehalogenating bacteria demonstrated that, under the selected conditions, 8 of the 10 minerals tested were able to maintain the pH in the appropriate range for PCE degradation, i.e between 5.5 and 6.5. However, the experimental results also revealed a mineral-induced potential inhibitory effect on dehalogenating activity for some minerals. It was shown that aluminum-containing mineral such as nepheline induced precipitation of phosphate which was detrimental to dehalogenating bacteria. Other sources of toxicity might have been the accumulation of toxic trace elements and the increase of the redox potential occurring during mineral dissolution. The bacterial species responsible for the transformation of DCE to ethene (Dehalococcoides spp.) appeared more sensitive than those responsible to reduce PCE to DCE. In conclusion, these study show that silicates mineral have a strong buffering potential, however, the choice of a mineral with a suitable composition is crucial to avoid inhibitory effect. Calcium and magnesium silicates appeared as interesting buffering agent.

2012

Utilization of Silicate Minerals for pH Control during In Situ Bioremediation of Chlorinated Solvents

David Andrew Barry, Alessandro Brovelli, Christof Holliger, Elsa Marina Lacroix

2012

Tide-induced surface water and groundwater interactions in coastal wetlands

David Andrew Barry

2011