Page 95 - ro membanes

78 4. SALINE WATER INTAKES AND PRETREATMENT
measure of the velocity of water movement through the ground (typically measured in m/s); the average specific yield (productivity) of the aquifer (in m3/day per linear meter of river- bank or seashore along which the collector wells are located); the thickness of the production aquifer deposits and associated well depth; the source water quality of the subsurface intake to determine the desalination plant pretreatment; fatal flaws for use of subsurface intakes such as severe beach erosion and damaging seismic activity; sources of subsurface contami- nation in the intake area; and the existence of nearby fresh or brackish water aquifers, which could be negatively impacted by the intake well operations or may have measurable effect on intake well-water quality.
A confined aquifer (also referred to as an artesian aquifer) is a water-saturated geological formation between two layers of low permeability (i.e., bedrock), which restrict the vertical movement of the groundwater in or out of the aquifer. Confined aquifers are often pressur- ized by the surrounding geological formations and, therefore, collecting water from such aquifers may not require pumping. Unconfined aquifers are groundwater-saturated forma- tions with fluctuating water level/table. Such fluctuation is driven by recharge from surface runoff (rain or snowmelt) or changes in the water table of surface water body (ocean, river, lake, etc.) hydraulically connected to the aquifer.
Coarse-grained porous and highly permeable geological formations (i.e., sandstones, beach sand and alluvial deposits, coarse-grained gravel and limestone) connected to a brackish riverbed (for brackish plant intakes) or to the ocean floor (for seawater intakes), whose specific yield (transmissivity) exceeds 1500 m3/day m, and which have water-carrying zone of at least 6.0 m (20 ft) are most suitable for subsurface intakes. The higher the aquifer permeability, trans- missivity, and thickness, the larger well yield the aquifer can support. Such soil conditions often exist along coastal dunes, reefs, and alluvial deltas. Fractured rock formations could deliver high volume of flow, but often the collected water is of inferior quality. The worst subsurface substrates for installation of subsurface intakes are those formed as a result of volcanic activity such as basalt and lava, as well as granite and clay formations.
One key criterion whether vertical wells can be used or horizontal wells would need to be installed is the presence of faults along the coast in parallel to the ocean shore. If such faults exist, typically, there is no hydraulic connection between the coastal aquifer and the sea and, therefore, vertical wells will not be suitable.
4.2.5.2 Need for Additional Pretreatment of Subsurface Intake Water
As mentioned previously, subsurface intakes typically yield better source-water quality than open intakes in terms of saline water turbidity and silt density index, which are two of the key parameters associated with the selection, sizing, and costs of the desalination plant pretreatment system. Therefore, often it is assumed that the use of subsurface intakes would eliminate the need for saline source water pretreatment prior to RO desalination.
However, the existing experience with the use of subsurface intakes for seawater desalina- tion in California and at the largest beach-well seawater desalination plant on the West Coast in Salina Cruz, Mexico, indicate that some desalination plants using subsurface intakes may face a costly challengedhigh concentrations of manganese and/or iron in the intake water. Unless removed ahead of the RO membrane system, iron and manganese may quickly foul the cartridge filters and RO membranes and render the desalination plant inoperable. The treatment of subsurface intake water that contains high concentrations of iron and/or