Evaporation Pond Seepage Soil Solution

from the soil surface. The subcores were fitted and sealed with plexiglass ends and set up to measure Permeability. Drainage water having an electrical conductivity (EC) of 10 dS/m (6100 ppm total dissolved salts) was applied to the cores for three days to ensure saturation and uniform electrolyte concentration. Biological activity was minimized in some of the cores by the addition of chlo- roform to the percolating drain water. Percolating drainage water having progressively larger EC values was applied over periods of one to five days in an effort to exaggerate variations in evaporation pond salinity resulting from evaporation lnfiltrometers (left)were installed in Kings County and fresh drain water additions. The sa- evaporation pond to estimate seepage. Rainfall, evaporation, drainage flows, and changes in pond linity of inflow and outflow water was water levels (herebeing checked by co-author Blake measured periodically along with the per- McCullough-Sanden)were also measured. meability of each subcore. The sodium adsorption ratio (SAR) is an index of the relative concentration of sodium, calcium, and magnesium in the Evaporation pond seepage soil solution. When soil salinity is low, permeability has been shown to increase Mark E. Grismer o Blake L. McCullough-Sanden as the SAR value of the inflow solution increases. Past studies, however, have typically considered SAR values of 30 or less. Rates of seepage from operating evaporation In this study, SAR values of the inflow so- ponds decline substantially as they age and as lution increased in the same stepwise fashion as EC, with values ranging from salinity increases 210 to 660. Field measurements S ince Kesterson Reservoir was closed waters and bed materials, and microbial Infiltrometers 6 inches in diameter and nearly two years ago, attention has fo- activity within or near the bed. Other fac- 40 inches tall were installed in an evapora- cused on local evaporation ponds for dis- tors affecting seepage rates, such as prox- tion pond in Kings County approximately posal of subsurface drainage water. Sev- imity to unlined canals and buried stream seven months after construction and fill- eral such ponds exist in the San Joaquin channels, also may be important. ing. Originally, 24 groups of three infil- Valley with a total acreage far exceeding The primary measurable factor control- trometers were installed on a 500- by 400- that of the 1200-acre Kesterson Reservoir. ling seepage unique to each pond is the foot grid spacing. This grid arrangement Although each local evaporation pond is permeability of the bottom soils. Al- was used to estimate pond seepage rates different, they all have features of concern though effects of salinity on soil permea- during the winter months of November to environmental interests. bility have been studied, these effects vary 1985 through February 1986. Drainage water evaporation ponds are greatly with the chemistry and concentra- In May, the 24 groups were replaced designed to receive and hold subsurface tion of the saline water. Such experiments with 90 individual infiltrometers set on a drainage water until it evaporates. As the have typically used salt concentrations 200-foot grid pattern to get a clearer pic- water evaporates, chemical constituents much lower than those encountered in ture of the change in seepage rates in are progressively concentrated, including evaporation ponds. Effects of microbial space across the pond. This second grid some that are potentially hazardous, such activity on permeability of soils have been arrangement was used during the sum- as arsenic and selenium. examined under a variety of conditions, mer months of May through August 1986. Primary concerns in evaluating the ef- and generally permeability decreases Seepage was also estimated by a vol- fectiveness of evaporation ponds as envi- with time of inundation under nonsterile ume mass balance, where rainfall, evapo- ronmentally acceptable methods of drain- conditions. ration, changes in pond water levels, and age water disposal are related to the ef- The purpose of this study was to assess drain flows into the pond were measured. fects of the ponds on wildlife and degra- the effects of salinity and time of ponding The depth of water required to balance dation of adjacent waters by seepage. on estimates of seepage from evaporation these components provides an estimate of Seepage of concentrated chemical con- ponds. We took both laboratory and field total seepage. Seepage rates by this stituents to adjacent groundwater or irri- measurements in the study. method were determined for winter, gation supply canals makes the evapora- spring, and summer periods. tion pond a point source of pollution simi- Laboratory measurements lar to other waste impoundments subject Large, 6-inch-diameter, undisturbed Permeability and salinity to regulation. soil cores were collected from five opera- Data collected from the nearly sterile Several factors may affect seepage from tional evaporation ponds. Several of the and the nonsterile cores varied, although evaporation ponds: the permeability of cores were divided into subcores 3 inches there were identifiable trends in drain pond bed materials (usually on-site com- by 3 inches in diameter taken at depths of water salinity and biological activity. pacted soils), interactions between pond 0.5 to 3.5,5 to 8,9 to 12, and 13 to 16 inches Generally, as time of ponding and drain 4 CALIFORNIAAGRICULTURE. JANUARY-FEBRUARY 1988 3 80 4- 120 - 100 60 3- 2 -. -80 - P .E % .E. n v) 40 p a -60 2 E 2- 0 -E -E Y' y1 :40 I2 20 1- - 20 04 . , . , . , . , 1 , . Lo 04., . , . , , , . , . $., . , ,Lo 0 100 200 300 400 500 600 0 100 200 300 400 500 600 700 800 900 TIME (hrs) TIME (hrs) 8 r 120 +K- * EC -100 - 100 6 -80 -80 r .E. .E. 0 v) 24 -60 p -60 3 -E Y :40 12 :40 1:: 2 - 20 - 20 0 to 0 .,.,.,.,.,.,.,.I. 0 0 100 200 300 400 500 600 700 800 900 0 100 200 300 400 500 600 700 800 900 TIME (hrs) TIME (hrs) Fig. 1. When percolating drain waterwas nonsterile (A and B), permeabil- decrease in salinity (B). When drain water was nearly sterile (C and D), ity, K, decreased as mean salinity, EC, increased. Permeability declined permeability tended to decrease with time (C) and showed only tempo- irreversibly as time of ponding increased (A) but rose again in relation to a rary, erratic changes at higher salinity levels. water salinity increased, permeability salinity effects on permeability is related such increases probably had a minor ef- decreased. Average salinity in figure 1 to microbial activity. fect on seepage rates from the pond. was determined from solution displace- Based on the laboratory experiments, ment considerations and represents the seepage from evaporation ponds should Seepage from evaporation ponds average EC of the core when measure- decrease with time of continuous Seepage from drainage water evapora- ments of permeability (K) were taken. ponding and increasing pond water salin- tion ponds is of concern to both environ- Under nonsterile conditions, permea- ity. Microbial activity at the pond bottom mental and agricultural interests because bility decreased substantially as salinity may result in some fluctuations in seep- of potential degradation of adjacent increased (fig. 1A and B). There also ap- age rates as pond water salinity increases groundwaters. From laboratory and field peared to be a reduction in permeability and decreases with evaporation and addi- experiments, it appears that rates of seep- simply due to the time of ponding (fig. tion of fresh drainage water. age from operational evaporation ponds 1A). As drain water salinity decreased, decline substantially after construction. permeability tended to increase and re- Measurements of pond seepage This decline is irreversible during con- turn to its original rate at the start of the During the winter, the average seepage tinuous ponding and may be a result of experiment. Increasing permeability rate measured by mass balance was 5.83 microbial activity and deposition of sus- with decreasing salinity is particularly feet per year, and decreasing trends in pended matter in pond water. Seepage evident in figure 1B. Overall, when bio- seepage were not detected. During the rates also decrease as the salinity of the logical activity is present, it appears that spring and summer, however, the average pond water increases as a result of chemi- the length of time of ponding causes an ir- seepage rate declined to 3.76 feet per year, cal precipitation and reaction with pond reversible decrease in permeability. or 36 percent less than the winter rate. bed materials, although this process may However, decreased permeability caused Seepage rates for most infiltrometers be partially reversible. Moreover, addi- by increasing salinity appears somewhat proved to be similar to the seepage esti- tional measurements of groundwater ele- reversible when salinity decreases. mated by mass balance. However, infil- vation and quality adjacent to and below When biological activity was reduced, trometers installed in areas of the pond the pond indicate that, when operational, permeability tended to decrease with bottom having coarse-textured soils interceptor drains surrounding the pond time of ponding and was relatively insen- showed decreases of 500 to 700 percent in partially contain seepage and confine it to sitive to changing drainage water salinity seepage. the area below the pond. (fig. 1CandD).Resultsshownin thesefig- Over the entire monitoring period, ures also are typical of the variability in pond water salinity ranged between 12.0 Mark E. Grismer is Assistant Professor, and permeability associated with controlling and 23.4 dS/m, with the minimum EC oc- Blake L. McCullough-Sanden is Postgraduate biological activity. A comparison of re- curring in December. Pond water EC Researcher, Department of Land, Air and Wa- sults shown in figure 1Aand B with those gradually increased within this range ter Resources, University of California, in C and D suggests that reversibility of during the summer months, although Davis.

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