Groundwater as a feedwater source for membrane treatment plants: Hydrogeologic controls on water quality variation with time

Abstract

A major misconception in the design of membrane water treatment facilities is that if a groundwater source is used to feed the plant, water quality will be relatively stable with time. There are wide variations in the rate of groundwater quality changes caused by pumping aquifers based on the local geology and hydrology. Brackish groundwater sources tend to show increases in the total dissolved solids concentration with time, while both brackish and seawater groundwater sources can have changes in the chemistry of the feedwater from basic dilute or standard seawater to water enriched in specific cation concentrations, particularly with iron or manganese. Modeling of these possible water quality changes should be accomplished prior to the final design of any membrane treatment facility.The primary geologic controls on groundwater quality within a subsurface aquifer system are the natural barriers which control the vertical inflow of water from adjacent aquifers containing water with different chemistries. Since the pumping rate of production wells is greater than the natural horizontal recharge rate in virtually all semi-confined aquifer systems, aquifer flow under an equilibrium condition is predominantly vertical through the confining beds. Therefore, the confining bed leakance coefficient along with other aquifer hydraulic characteristics, and the pattern and rate of pumping tend to control the rate of water quality change within any production aquifer. This water quality change is caused by mixing of the leaked water with the natural water contained in the production aquifer.Aquifer and confming bed materials tend to control the type of chemical composition changes in the water yielded to the wells with time. In limestone aquifers separated by compacted lime muds, such as in Florida, there are few changes in water chemistry outside of dilute seawater type compositions with the possible exception of sulfate and hardness. In clastic aquifers separated by clay mineral confining beds, there is a tendency for enrichment of the feedwater in dissolved iron or manganese. In fractured rock aquifers within igneous or metamorphic rocks, a wide range in water chemistry changes can occur.Most significant water quality changes can be predicted to some degree of accuracy based on the hydrogeologic test program conducted prior to wellfield design. If the geology is known, the relevant aquifer characteristics are measured, and the initial water quality in the aquifer above and below the production aquifer are measured, then changes in water quality with time can be modeled. Since the long-term water quality changes can be controlled to a large degree by the design of the feedwater wellfield, the economics of both water treatment and plant construction can also be significantly altered by well spacing and pumping rates. The accuracy of predictive solute transport modeling is directly related to the availability of data on the aquifer system modeled. Large errors in the modeled water quality changes can occur when data are insufficient. When significant restrictions on data occur, a very conservative wellfield design approach is prudent.