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[1] A regional scale hydrogeologic model has been developed to estimate the magnitude of submarine groundwater discharge to the coastal waters of southeastern Louisiana. The model domain incorporates both the onshore recharge area of terrestrially derived freshwater, and fluid circulation within the sediments on the continental shelf. The hydrogeologic properties of these sediments, which form part of the Coastal Lowlands Aquifer System, have been well-characterized in earlier studies. The low topographic relief of the coastal plain and an extensive zone of seawater intrusion are key features of the groundwater flow system. Model calculations suggest that no water containing a substantial component of terrestrial origin discharges on the continental shelf. Rather the near-shore coastal zone serves as a groundwater recharge area of saline water that then forms the seawater recirculation system beneath the coastal plain. The modeling results are consistent with interpretations of the rates of submarine groundwater discharge derived from geochemical tracers, presented in a companion paper by McCoy et al. (2007).

At sites where a dense nonaqueous phase liquid (DNAPL) was spilled or released into the subsurface, estimates of the mass of DNAPL contained in the subsurface from core or monitoring well data, either in the nonaqueous or aqueous phase, can be highly uncertain because of the erratic distribution of the DNAPL due to geologic heterogeneity. In this paper, a multiphase compositional model is applied to simulate, in detail, the DNAPL saturations and aqueous-phase plume migration in a highly characterized, heterogeneous glaciofluvial aquifer, the permeability and porosity data of which were collected by researchers at the University of Tübingen, Germany. The DNAPL saturation distribution and the aqueous-phase contaminant mole fractions are then reconstructed by sampling the data from the forward simulation results using two alternate approaches, each with different degrees of sampling conditioning. To reconstruct the DNAPL source zone architecture, the aqueous-phase plume configuration, and the contaminant mass in each phase, one method employs the novel transition probability/Markov chain approach (TP/MC), while the other involves a traditional variogram analysis of the sampled data followed by ordinary kriging. The TP/MC method is typically used for facies and/or hydraulic conductivity reconstruction, but here we explore the applicability of the TP/MC method for the reconstruction of DNAPL source zones and aqueous-phase plumes. The reconstructed geometry of the DNAPL source zone, the dissolved contaminant plume, and the estimated mass in each phase are compared using the two different geostatistical modeling approaches and for various degrees of data sampling from the results of the forward simulation. It is demonstrated that the TP/MC modeling technique is robust and accurate and is a preferable alternative compared to ordinary kriging for the reconstruction of DNAPL saturation patterns and dissolved-phase contaminant plumes.

[1] Contaminant loading from an inland source zone to the nearshore marine environment is examined using a saturated-unsaturated simulation model which incorporates both the influence of the growth and the extinction of the seepage face in the intertidal zone and the inclusion of hydromechanical stress coupling due to tidal loading. The system analyzed represents a site where an unconfined aquifer extends offshore with a broad intertidal zone. The volumetric discharge of groundwater in the intertidal zone dominates the discharge occurring beyond the low-tide line. For most of the cases examined, within the intertidal zone, the recirculated saline component of the discharge is greater than the freshwater component. The majority of the contaminant mass exits through the seepage face located above the instantaneous tide line. The peak contaminant loading rate occurs around the time of the falling midtide. At low tide, the contaminant loading rate is more than 2 orders of magnitude lower than the peak rates, even though groundwater discharge rates are highest as low tide is approached. Sensitivity studies indicate that the relative proportion of recirculated intertidal zone discharge to fresh intertidal zone discharge is most sensitive to the magnitude of the freshwater flux, the spatial variation of the outflow across the intertidal zone as influenced by anisotropy in hydraulic conductivity, and tidal pumping as influenced by specific storage. Dilution of contaminant concentrations due to mixing with seawater beneath the sediment-water interface is represented in terms of a dilution factor, defined relative to the onshore source concentration. For our base case, the dilution factor for the contaminant concentration averaged over one tidal cycle at the high-tide, midtide, and low-tide lines was 1.3, 3.9, and 560, respectively. The key factors determining contaminant dilution at the sediment interface are location within the intertidal zone and the vertical extent of the contaminant plume as it enters the intertidal zone.Citation: Maji, R., and L. Smith (2009), Quantitative analysis of seabed mixing and intertidal zone discharge in coastal aquifers, Water Resour. Res., 45, W11401,

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