Ann E. Mulligan

Seasonal oscillations in water exchange between aquifers and the coastal ocean.

Ground water of both terrestrial and marine origin flows into coastal surface waters as submarine groundwater discharge, and constitutes an important source of nutrients, contaminants and trace elements to the coastal ocean. Large saline discharges have been observed by direct measurements and inferred from geochemical tracers, but sufficient seawater inflow has not been observed to balance this outflow. Geochemical tracers also suggest a time lag between changes in submarine groundwater discharge rates and the seasonal oscillations of inland recharge that drive groundwater flow towards the coast. Here we use measurements of hydraulic gradients and offshore fluxes taken at Waquoit Bay, Massachusetts, together with a modelling study of a generalized coastal groundwater system to show that a shift in the freshwater–saltwater interface—controlled by seasonal changes in water table elevation—can explain large saline discharges that lag inland recharge cycles. We find that sea water is drawn into aquifers as the freshwater–saltwater interface moves landward during winter, and discharges back into coastal waters as the interface moves seaward in summer. Our results demonstrate the connection between the seasonal hydrologic cycle inland and the saline groundwater system in coastal aquifers, and suggest a potentially important seasonality in the chemical loading of coastal waters.

Advective control of groundwater contaminant plumes: Model development and comparison to hydraulic control

A new optimization formulation for designing groundwater plume control systems is presented. The new formulation uses particle-tracking techniques in a two-step solution process. The two-step procedure is motivated by numerical and computational considerations; particle representation is defined to take advantage of specific properties and improve model convergence. The optimization formulation seeks the least cost control system that satisfies the two equivalent requirements that the contaminant plume be located within the capture zone (step 1) and that all particles representing contaminant solute travel to an extraction well (step 2). To date, optimization formulations for plume capture design have emphasized either hydraulic or concentration control; however, these formulations provide indirect representation of the plume control and containment problem. The model presented here explicitly represents the capture zone design problem using particle tracking and formalizes the design procedures used by many practitioners. Two example problems representing two- and three-dimensional flow systems are used to demonstrate the new advective control model. Hydraulic control formulations for the two problems are also developed, and designs are compared with those of the advective control model. Control systems resulting from the hydraulic control model are sensitive to constraint magnitude and location, highlighting the need for constraint calibration in order to best achieve design goals. Conversely, constraints in the new model directly represent the plume capture problem, and the model provides more efficient capture zone designs than the hydraulic control formulation.

Groundwater Flow to the Coastal Ocean

Significant freshwater input and associated chemical loading to coastal waters can occur via submarine groundwater discharge (SGD). Although such inputs have historically been ignored, recent work has revealed that SGD can be a major input to coastal waters and can account for a significant portion of chemical inventories. Because SGD has been only recently recognized as an important process in coastal water dynamics, our understanding of the physical and chemical processes that control groundwater flow and chemical fate and transport in coastal regimes is limited. A critical step in any SGD study is the determination of groundwater flow volume, timing, and location, and the associated chemical loading. Significant spatial and temporal variability in flow, solute concentration, and biogeochemical processes complicate such studies. Here we review physical and chemical means for estimating groundwater discharge rates for both fresh groundwater and recirculating seawater.

A Differential Pressure Instrument with Wireless Telemetry for In-Situ Measurement of Fluid Flow across Sediment-Water Boundaries

An instrument has been built to carry out continuous in-situ measurement of small differences in water pressure, conductivity and temperature, in natural surface water and groundwater systems. A low-cost data telemetry system provides data on shore in real time if desired. The immediate purpose of measurements by this device is to continuously infer fluxes of water across the sediment-water interface in a complex estuarine system; however, direct application to assessment of sediment-water fluxes in rivers, lakes, and other systems is also possible. Key objectives of the design include both low cost, and accuracy of the order of ±0.5 mm H2O in measured head difference between the instruments two pressure ports. These objectives have been met, although a revision to the design of one component was found to be necessary. Deployments of up to nine months, and wireless range in excess of 300 m have been demonstrated.

New interpretation of glacial history of Cape Cod may have important implications for groundwater contaminant transport

Fresh water resources of sufficient quantity and quality are critical for maintaining societies and for supporting additional growth and development. When these resources are threatened or compromised, as can occur through the release of hazardous compounds, additional stress is placed on the water supply system from loss of the resource and changes in the demand structure. In western Cape Cod, Massachusetts, such problems are currently being encountered as a result of contaminant releases from the Massachusetts Military Reservation (MMR). An effective long-term response to subsurface contamination requires, among other things, determining the lithology, stratigraphy, and structure of aquifer materials and their effects on groundwater flow and contaminant transport. A recent review and analysis of subsurface data across Cape Cod offers a new interpretation of the geologic history of the Cape, with potential implications for groundwater issues facing western Cape Cod (the Upper Cape).

A New Interior-Point Boundary Projection Method For Solving Nonlinear Groundwater Pollution Control Problems

A new interior-point algorithm for solving the groundwater-pollution-control design problem is presented. The algorithm requires that the objective function is differentiable in the interior region. For minimization problems with nonlinear constraints and a concave objective function, the technique is shown to be similar to an active set gradient-projection method, where the tangent of the boundary between feasible and infeasible solutions is used to determine a search direction. In this new method, however, the search direction is translated into the interior space of the feasible region. This process allows progress to be made toward improving the objective function while remaining in the feasible space and ultimately converges to a stationary point. Although the solution technique was developed to solve a groundwater control formulation with a linear objective function and nonlinear constraints, the method has been successfully applied to an unconstrained nonconcave/nonconvex formulation and may be applicable to a wide variety of problems.

Optimal Plume Capture Design in Unconfined Aquifers

A combined simulation and optimization model for designing groundwater plume control systems in unconfined aquifers is presented and demonstrated on two- and three-dimensional aquifer problems. Unconfined aquifer simulation poses numerical challenges that are not present when confined conditions are assumed. In unconfined aquifers, excessive extraction from a pumping well may result in drying of the well and cessation of pumping. In the simulation model, excessive extraction rates result in dewatering portions of the numerical domain. To avoid numerical dewatering, constraints can be placed on hydraulic head within the well cell. However, head is a nonlinear function of pumping in unconfined aquifers. The nonlinearity of head and the potential for dewatering must both be considered when applying simulation-optimization models to design containment systems in unconfined aquifers. In this chapter, optimization search procedures for an advective-control model are developed for accommodating unconfined aquifer simulation. The simulation-optimization model represents advective contaminant transport using particle tracking techniques. The goal of the groundwater management strategy is to contain plume migration while minimizing total pumping of the containment system.