Effects of Near-shore Groundwater Circulation on Aerobic Biodegradation in Coastal Unconfined Aquifers

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1 First International onference on Saltwater Intrusion and oastal Aquifers - Monitoring, Modeling, and Management. Essaouira, Morocco, April 3, 1 Effects of Near-shore Groundwater irculation on Aeroic iodegradation in oastal Unconfined Aquifers L. Li 1,, P. Enot 1, H. Prommer, F. Stagnitti 3, and D. A. arry 1, 1 School of ivil and Environmental Engineering, University of Edinurgh, United Kingdom ontaminated Land Assessment and Remediation Research entre, University of Edinurgh, United Kingdom 3 School of Ecology and Environment, Deakin University, Australia ASTRAT Wave set-up induces groundwater circulation in the near-shore area, which promotes exchanges etween oxygen-rich seawater and groundwater, and may create an active zone for aeroic acterial populations in the phreatic aquifer. MDFLW and PHT3D were used to model contaminant transport and iodegradation in coastal aquifers affected y wave set-up. The results showed that the oxygen concentration in the near-shore active zone is high compared with the oxygen level of the inland groundwater. Aeroic acterial activity sustained y the high oxygen concentration degrades the contaminants, leading to a considerale reduction in reakthrough concentrations of aeroic iodegradale contaminants at the ocean-aquifer interface. These effects, largely ignored in previous studies, may have significant implications for each environment. 1. INTRDUTIN Groundwater contamination is ecoming a common prolem in coastal zones due to industrial and agricultural developments. The contaminants travel seaward with the groundwater and may cause considerale degradation of the littoral environment. For example, marine species that inhait the each face are sensitive to local chemical concentrations [ampell and ate, 1998]. Therefore, the exit contaminant concentration at the each face is an important parameter in assessing the impact of groundwater contamination on the marine environment. In particular, since strong mixing with amient seawater will occur once the contaminant enters the coastal sea, the exit concentration at the reakthrough point will e higher than elsewhere in the coastal zone. That is, for coastal ecosystems, this point likely presents the worst scenario in assessing risk associated with the contamination prolem. The same situation occurs for groundwater discharging into a tidal estuary. There, estuarine ecosystems involving groundwater discharge regions will e affected y contaminant concentrations at the estuary-aquifer interface. To determine the exit contaminant concentration at the each face, we need to understand and quantify the chemical transport and reaction processes in the coastal aquifer. An essential feature of a coastal aquifer is the influence of oceanic oscillations on flow and mixing processes in the mixing zone of aquifer (called suterranean estuary hereafter). Li et al. [1999] demonstrated using a simple ox

2 model that local groundwater circulation induced y wave set-up and tidally driven oscillating flows (Figure 1) can reduce the residence time of chemicals in the suterranean estuary, similar to tidal flushing of a surface estuary. As a result, the rates of chemical fluxes from the aquifer to the ocean can increase significantly and may impose threat to coastal marine environments. These results are consistent with field findings [Moore, 1996]. In this paper, we present a modelling study of the effects of wave induced groundwater circulation on aeroic iodegradation in coastal aquifers. land ocean water tale Fresh water wave set-up tide net groundwater flow wave set-up circulation tidal oscillations of rackish water zone pumping rackish water Figure 1. Schematic diagram of coastal groundwater flow processes.. RSS-SHRE MDELLING F AERI IDEGRADATIN IN ASTAL AQUIFERS We shall consider the transport process to e one-dimensional (i.e., depth averaged) in the cross-shore direction (Figure ). Vertical variations of flow and chemical properties are assumed to e of secondary importance. ur purpose here is to demonstrate possile effects of near-shore groundwater circulation on iodegradation in the aquifer. Furthermore, a vertical each face is assumed and the density effects on the flow are ignored. The near-shore groundwater circulation is simulated via recharge to several cells near the seaward oundary. Note that the circulation is an advective process and hence the averaging (over depth) should not lead to overestimates of oxygen enrichment in the near-shore zone. The aeroic iodegradation can e limited y either the iodegradale contaminant or the molecular oxygen. If oxygen is the limiting factor, the transport and reaction equations are [Kindred and elia, 1989], R Y o V = D V Yc mo ho I + (1a) = D V V mo ho I + (1)

3 d Vmo I = Yo K m dt ho (1c) + where R is the retardation factor; D is the dispersion coefficient; V is the flow velocity; is the concentration of contaminant; is the oxygen concentration; Y o and Y c are the yield coefficients for oxygen and contaminant, respectively; V mo and K ho are the kinetic parameters for oxygen uptake y acteria; is the aeroic iomass; K m is the maintenance constant; x is the cross-shore coordinate as defined in Figure ; t is time; and I is the iomass inhiition factor, and I = 1 + () k where k is the iomass inhiition coefficient. For the contaminant-limiting case, the governing equations are [Kindred and elia, 1989], V I mc R = D V (3a) hc + = D V Y c V Yo mc hc I + (3) d Vmc I = Yc K m dt hc (3c) + where V mc and K hc are the kinetic parameters for contaminant uptake y acteria. -rich circulating groundwater,, iomass () sea x Figure. 1D modelling of aeroic iodegradation in a coastal aquifer. In the hypothetical example shown elow, the concentrations of oxygen and contaminant at the inland oundary (LHS) are maintained constants. n the seaward side oundary (RHS), zero concentration gradients are applied. 3

4 3. SIMULATIN RESULTS AND DISUSSIN Simulations were conducted using MDFLW and PHT3D. PHT3D is a newly developed multispecies, reactive chemical transport model/program, which comines MT3D and PHREEQ [Prommer et al., 1999, ]. The values of transport and reaction parameters used in the simulations are listed in Tale 1. The initial concentrations of oxygen and iomass in the aquifer are 3 mg/l and. mg/l respectively. The concentrations of oxygen and contaminant at the landward oundary are 3 mg/l and 1 mg/l, respectively. The recharge rate due to groundwater circulation (over 3 m cross-shore distance) is. m/d. The oxygen concentration of the circulating water is set to e 11 mg/l (saturated oxygen concentration in seawater). For the given parameters, the iodegradation is predominantly oxygenlimited. Two simulations were conducted without and with near-shore groundwater circulation included. The spatial concentration profiles at various times are shown in Figure 3. The results from simulations without groundwater circulation (left panels in Figure 3) are similar to those presented y Kindred and elia [1989]. xygen supplied from the inland oundary is depleted very quickly. Therefore, oth the acteria growth and contaminant degradation are confined in area near the inland oundary. The front of the contaminant plume moves towards the sea and reakthroughs at the ocean-land interface with a rather high concentration (despite the initial degradation). The groundwater circulation creates an oxygen-rich area near the shore and leads to acteria growth (Figure 4). As a result, the contaminant is degraded further. The reakthrough concentration is reduced to 7.1 mg/l compared with 8. mg/l in the case of no groundwater circulation (Figure ), a considerale reduction (aout 16%). 1 t = 1 days t = days t = 1 days x (m) t = 1 days t = days t = 1 days x (m) Figure 3. Spatial concentration profiles from simulations without (left panels) and with groundwater circulation (right panels). 4

5 .8.7 increased increased iomass.1 low iomass depleted x (m) Figure 4. Increased oxygen concentration and iomass growth in the near-shore area due to groundwater circulation without circulation with circulation 6 oncentration (mg/l) t (days) Figure. omparison of the contaminant reakthrough curves from simulations without (dashed line) and with (solid line) groundwater circulation.

6 Tale 1. Parameter values used in the simulations Y o Y c K ho K hc V mo V mc k K m.1 L/mg. L/mg.1 mg/l.1 mg/l 1 d -1 1 d -1 1 mg/l.1 d -1 V = 1 m/d, D =. m /d, and R = 1. The results in Figure 4 show that the residual oxygen concentration of the discharging groundwater is low. Neglecting this low concentration (i.e., assuming 1% consumption of input oxygen), a simple solution for the contaminant reakthrough concentration can e derived ased on the mass alance principle and the stoichiometry of the kinetic reactions, ( V + V ) V l l c c ll t =, (4) Vl + Vc where V l is the inland groundwater flow rate and V c is the circulating flow rate; l and l is the inland contaminant and oxygen concentrations, respectively; c is the oxygen concentration of the circulating water; and is the stoichiometry constant. Given the parameter values used in the simulation and using (4), t is calculated to e 6.67 mg/l, slightly lower than the simulated value (i.e., overestimating the degradation). 4. NLUSINS We have demonstrated using a hypothetical example that wave induced groundwater circulation can lead to considerale reduction of reakthrough concentration of aeroic iodegradale contaminants at the each face. The effects of dynamic oceanic oundary conditions, ignored y previous studies on contaminant transport in coastal aquifers, could well have important implications for risk assessment of coastal and estuarine pollution. To ascertain these effects, further studies, especially experimental investigations, need to e carried out. References ampell, E. E., and G.. ate, 1998, Tide-induced pulsing of nutrient discharge from an unconfined aquifer into an Anaulus australis-dominated surf-zone, Water SA, 4, Kndred, J. S., and M. A. elia, 1989, ontaminant transport and iodegradation: onceptual model and test simulations, Water Resour. Res.,, Li, L., D. A. arry, F. Stagnitti, and J.-Y. Parlange, 1999, Sumarine groundwater discharge and associated chemical input to a coastal sea, Water Resour. Res., 3, Moore, W. S., 1996, Large groundwater inputs to coastal waters revealed y 6Ra enrichment, Nature, 38, Prommer, H., Davis, G.., and arry, D. A., Geochemical changes during iodegradation of petroleum hydrocarons: field investigations and modelling. rg. Geochem., 3,

7 Prommer, H., arry, D. A, and G.. Davis,. Numerical modelling for design and evaluation of groundwater remediation schemes. Ecol. Model., 18, Keywords: Hydrogeochemistry, aeroic iodegradation, coastal aquifer, near-shore groundwater circulation, numerical modeling, PHT3D. orresponding author: Dr. Ling Li, Lecturer in Environmental Engineering, School of ivil and Environmental Engineering, University of Edinurgh, EH9 3JN, UK, 7