Water Well Journal
Issue link: https://read.dmtmag.com/i/557876
carrying increased quantities of sand into the well. If a portion of the well screen is dewatered due to declining groundwater levels, the remaining satu- rated screen will experience increased entrance velocities (assuming the well's pumping rate remains the same). Groundwater quality is stratified in many areas, just as alluvial sediment is stratified. An aquifer may have a variety of man-made or naturally occurring contaminants at different depths, and the resulting water quality of the well will reflect the proportionate contributions from the various depth intervals that have been penetrated. An example is a well with elevated nitrate (NO 3 ) in the shallow portion of the aquifer, and elevated arsenic (As) in the deeper portion of the aquifer. The well will produce a blend of the shallow groundwater (high in NO 3 , but low in As) and the deeper groundwater (high in As, but low in NO 3 ). In many cases, the in-well blending of water produced from both the shal- low and deep strata of the aquifer will result in good quality water. However, if drought conditions or over-pumping cause the water table to drop, that same well might begin to produce water only from the deeper aquifer, so the concentration of arsenic may become excessive. The impacts of declining water levels can often be addressed with a flexible well design, so that as conditions change over time, the well structure can be modified to accommodate the changed conditions. A time-sequence example of a well with declining water levels is shown in Figure 3, assuming a single aquifer with no water quality issues. As the aquifer progresses from fully saturated to mostly dewatered, the unsaturated por- tions of the well's screened interval can be blanked off with liners to prevent any perched aquifers from causing cascad- ing or poor-quality water to enter the well from above the water table. This solution is possible because the well was initially designed with bentonite seals in the annulus to isolate the screened intervals in anticipation of a drop in the water table. The longevity of any well is finite, so changed conditions can sometimes be addressed by altering the design of re- placement wells in future years. How- ever, this can be problematic in urban areas due to the limits of available land. If there is a restriction of land avail- ability for well sites, flexibility of well design within the same borehole can sometimes be accomplished by use of an intermediate casing. As shown in Figure 4, a heavy-walled, corrosion- resistant steel casing with a sufficiently large diameter can be initially cemented into the well, just above the screened interval. The wall thickness and steel type of the intermediate casing must be adequate for it to last many years. The intermediate casing must have a large DROUGHT from page 17 Figure 3. Example of a well designed to accommodate a declining water table. Figure 2. Historic land subsidence (1925- 1977) in San Joaquin Valley, California. waterwelljournal.com 18 September 2015 WWJ DACUM Codes To help meet your professional needs, this article covers skills and competen- cies found in DACUM charts for drillers and pump installers. DO refers to the drilling chart. The letter and number immediately following is the skill on the chart covered by the article. This article covers: DOA-3, 4; DOB-1, 2; DOE-4, 8, 9, 12, 16; DOF-1, 2; DOG-6, 9 More information on DACUM and the charts are available at www.NGWA.org.