Vertical wells are identical to conventional groundwater production wells. Typically a series of vertical wells would be drilled along a beach location, and screened within unconsolidated beach sand and alluvium. Pumping would induce flow of both seawater and inland groundwater to the wells.
Slant wells are drilled at an angle from the shore toward the sea. This potentially allows the well screen to be beneath the sea floor, which can more effectively induce vertical flow from the ocean into the well. However, because the well screen may be hundreds of feet below the sea the hydraulic connection to the sea may be limited.
HDD wells can be installed beneath the seafloor from the shoreline. During the drilling of the pilot boring the drill operator can maneuver the drill head, gradually directing it to the desired stratum or location. This is different from a slant well, where the borehole is drilled straight (e.g. ARCADIS, 2013). Typically the boring would exit the seafloor and the casing would be pulled back into the boring from offshore. Groups of HDD wells can fan out at shallow depths beneath the seafloor from a common location inland of the beach. Installation of a filter pack is generally not possible (e.g. ISTAP, 2014). HDD wells are typically relatively small diameter and limited to lengths 1000 feet to 2000 feet. However, pipelines with diameters of a couple feet and length of several thousand feet have been installed using HDD beneath rivers (Iseley and Gokhale, 1997). Several HDD wells can be drilled in clusters from a single location, therefore reducing the land area required for SSIs along the coastline (Missimer et al., 2013).
Radial collector wells (e.g. Ranney WellsTM) include a central caisson, typically having a diameter of 10 to 20 feet (ISTAP, 2014) that extends down into the sand to a depth typically in the range of 30 to 150 feet. Horizontal lateral wells fan out from the caisson to distances of 200 to 300 feet. Radial collector wells are commonly used for large-scale water production beneath rivers (e.g. Missimer et al., 2013).
Beach infiltration gallery (BIG) intake systems are constructed beneath the intertidal zone of the beach. A BIG consists of a network of perforated pipes typically placed beneath the beach and typically covered with series of sand layers that increase in grain size with depth. The top layer is the native beach sand and the lowest layer is gravel. Seawater percolates through the sand and is pumped from a header pipe connected to a network of perforated pipes of well screens (ISTAP, 2014). The mechanical energy of breaking waves in the intertidal zone may continuously clean the overlying sand layer (Missimer et al., 2013).
A seabed infiltration gallery (SIG) is constructed offshore at a stable location and like a beach infiltration gallery includes a network of perforated pipes or screens covered by engineered fill. A SIG should provide slow sand filtration and the uppermost layer is contributes most to treatment of the infiltrating water (ISTAP, 2014).
A deep infiltration gallery (DIG) or water tunnel is a large pipe or tunnel beneath the sea floor that connects a series of vertical or radial collector wells to an onshore pump station. Figure 7 below shows a DIG that consists of two concentric pipelines with the inner pipeline serving for brine discharge.
In the situation where a subsurface intake is not feasible for a proposed project the California State Water Resources Control Board(SWRCB) adopted Amendments to the California Ocean Plan requiring a 1.00mm maximum slot opening for a screened intake. These intakes are cylindrical screens, sometimes called wedge wire screens due to the shaper of the wires wound around the frame to create the screen, and come in a variety of sizes and materials. The screens minimize approach velocities into the screen so marine life can swim away and not get impinged on the screen itself. The slot sizes of 1.00mm keep out all adult, juvenile and most, if not all mature larvae in the ocean, protecting the reproductive marine life and the future of the ocean.
West Basin has tested several wedge wire screens by different manufacturers and with different slot sizes to quantify I&E through the screens. West Basin operated an Ocean Water Desalination Demonstration Facility in Redondo Beach, CA from 2010-2014 to identify and quantify all intake impacts. Through the report and 1.5 year study West Basin determined there was zero impingement on the screen due to the low intake and approach velocities of 0.33 ft/second. This intake velocity is well below the Environment Protection Agency (EPA) impingement limit for power plant intakes of 0.5 ft/second and protecting more marine life. Entrainment was calculated by using the SWRCB approved method of the Empirical Transport Model (ETM) to determine a ratio of larvae entrained compared to larvae in the water column at the intake location. These calculations and findings per species can be found in the Intake Effects Assessment Report.
When ocean water is pumped through the Reverse Osmosis (RO) units of the desalination process, the fresh water molecules are separated from the other dissolved compounds (mostly salts) and this process is operated at what is called a 50% recovery. A 50% recovery means it takes two gallons of ocean water to produce one gallon of fresh drinking water and one gallon of twice as salty water, called brine. This brine is still a liquid, but with twice as much salt and other dissolved compounds as when it came in. The primary location to send the brine is back in the ocean. Many questions have been raised about the impacts of this twice-as-salty brine on the local marine environment.
West Basin has evaluated the impacts of the high salinity brine on marine organisms utilizing proven and regulator accepted methods of the Whole Effluent Toxicity (WET) test. This method requires evaluating marine organisms local to the area of discharge and changes to their behavior, growth, lengths, weight, survival, etc. The findings from West Basin’s High Salinity Sensitivity Study can be found here.
In order to reduce the impacts to the local marine environment the brine should be mixed as rapidly as possible to minimize any potential salty plume on the ocean floor. Brine discharge regulations in California have been put in place allowing two options as specified by the SWRCB in the Amendments to the California Ocean Plan. In the first option a project proponent must first determine the feasibility of utilizing an existing discharge or outfall structure with a waste water treatment plant. Most waste water treatment plants have ocean discharges and may be able to accept a brine stream from an ocean water desalination facility. However, not in all cases are desalination facilities located near a waste water treatment plants and have the ability to comingle discharges. In the instance a waste water comingled discharge was not feasible a brine diffuser system would be utilized.
A brine diffuser system is essentially a pipe going out to the ocean at a specified distance and depth of with several nozzle ports to discharge the water. These nozzles create a large amount of mixing similar to covering up part of a hose nozzle and allowing the water to spray out faster. The key to mixing the brine quickly is to limit the exposure time of marine organisms to the twice as salty brine. Modeling can be done to determine how large a of a bine plume and at what concentrations would be created. West Basin has modeled the brine discharge to determine within a matter of a few meters radially around the diffuser the brine is within 5% of the ambient salt levels. All of these models are run assuming no mixing from the ambient ocean currents and are very conservative as to quantify any potential impacts. The findings from West Basin’s Brine Diffuser Entrainment Study can be found here.