Subsurface Drainage Evaporation Ponds

Subsurface drainaae evaDoration Donds w I I Kenneth K. Tanji 0 Mark E. Grismer 0 Blaine R. Hanson

They offer some advantages if well designed and operated withthe scheduled closure of Kester- 20 percent of the land. Numerous factors In addition, assuming a subsurface son Reservoir as a collection basin for sa- influence the size of ponds. flux of 0.05 inch per day from upslope line subsurface drainage waters from Annual evaporation from surfaces of irrigated lands, lateral flow across a farms in the Westlands Water District, at- water bodies along the west side of the cross-sectional area of the farm yields tention has turned to on-farm and region- San Joaquin Valley is about 55 inches. about 55 acre-feet. The total quantity of al evaporation ponds as alternatives. With an average rainfall of about 6 inches drain water produced is then 223 acre- Some such ponds are already in existence, in the area, only about 49 inches of water feet per year for the 240 acres (0.9 acre- and their use for disposal of saline drain per year can be evaporated. The evapora- foot per acre of irrigated land; 0.7 acre- waters is likely to escalate until a drain- tion rate would decrease with increasing foot per acre for the entire 320 acres). age system for the San Joaquin Valley as salinity in the pond water. If one assumes Allowing about 5 feet per year for a whole becomes available. inadvertent seepage from a pond at an evaporation and seepage, the pond sur- Both short- and long-term efficacy of average rate of about 11 inches per year, face area required to evaporate 223 acre- evaporation ponds need to be appraised. the total depth of water that would be dis- feet per year is about 45 acres. Adding Problems of concern include possible posed of through evaporation and seepage land lost to the pond foundation, perim- seepage from ponds to groundwaters, the is about 60 inches per year. eter roads, and the like brings the total costs of pond construction and cropland We have estimated the pond surface area required for the pond to 50 acres, or taken out of production, and the potential area required for drainage from a 320- 15.6 percent of the 320-acre tract. accumulation in the ponds of toxic con- acre farm, of which 240 acres are irrigat- If the drain water collected in the pond stituents to hazardous levels. The State ed and the remainder is in roads, canals, has an electrical conductivity of 10 deci- Water Resources Control Board and its evaporation pond, or nonirrigated land. If Siemens per meter or 6,400 milligrams Regional Water Quality Control Boards, 3 feet of water are applied to a crop like per liter total dissolved solids, it would the University of California, US. Depart- tomatoes and the crop uses 2.3 feet, drain- contain about 8.7 tons total dissolved so- ment of Agriculture Soil Conservation age beyond the root zone would be 0.7 lids per acre-foot. The 223 acre-feet of Service, and others are conducting re- acre-foot per acre, assuming no surface drain water collected would contain about search to find environmentally accept- runoff. Thus, 240 acres of irrigated land 1,940 tons of total dissolved solids. As the able and efficient methods of disposal of would produce 168 acre-feet per year of pond water is concentrated by evapora- unusable saline drain waters. drain water. tive salinization, precipitation of various On-farm vs. regional evaporation ponds Carbon dioxide Water evaDoration On-farm evaporation ponds offer the Volatilization - discharger an immediate short-term solu- t tion and provide more opportunities for reuse of usable drain water than do re- \i Ion association I gional ponds. However, they remove land and complexation from production, construction costs must be borne by individual growers, maintenance and monitoring may be limited, Specific adsorption Oxidation- and seepage to adjacent lands and a reduction perched water table may occur. Clay fixation Regional evaporation ponds have the advantages of being built on marginal Microbial transformations Particle land, possibly of better design and con- Mineral sedirnentation- struction, and with better maintenance deposition-dissolution resuspension and monitoring. Their disadvantages are that they offer limited potential for drain- J water reuse, require an extensive collec- 1 Water and salt seepage tion system, and may cost more to build.

Pond size and salt accumulation Fig. 1. Numerous physical, chemical, and biological processes influence the complex chem- Land taken out of production for evap- istry of pond water: degassing of carbon dioxide, microbial transformation of sulfate to hy- oration ponds may be as little as 3 percent drogen sulfide with subsequent volatilization, ion association and complexation, cation ex- change involving replacement of exchangeable calcium from the soil by sodium from the of the land being drained, if the drainage pond water, adsorption of cadmium and fixation of potassium by soil clays, oxidation of water is reused, to as much as 33 percent organic carbon, reduction of nitrate to nitrogen gases, precipitation of calcite and gypsum with no reuse. Typical values used by con- with evaporative salinization, and sedimentation and resuspension of colloidal and larger sulting engineering firms are from 15 to particles.

IO CALIFORNIA AGRICULTURE, SEPTEMBER-OCTOBER 1985 types of minerals occurs in sequence. As brine in the evaporation pond dries up, mineral salts will be deposited, along with some brine in the void spaces between the crystallizing minerals. If the bulk density CaC03 of salt crusts in the pond is about 45 2mca > alkalinity 2mca < alkalinity pounds per cubic foot, the 1,940 tons of salt would produce a salt crust about 0.5 Na, Mg, Cog, SO4, CI inch thick per year in the 45-acre pond - 86,000 cubic feet. This rate of salt accumulation would produce, in 20 years, a Gypsum deposition aSepiolite deposition crust of about 10 inches weighing over CaS04 2H20 MgSi306(OH)2 38,800 tons. Pond water chemistry mca m~04 mCa < mSo4 2mMg > alkalinity 2mMg ialkalinity Numerous chemical, biological, and physical reactions and processes can take Na, Ca, Mg, CI Na, Mg, SO4, CI Na, Cog, SO4, CI place in a pond (fig. 1). The chemistry is 11 complex and would change with salinization because of evaporation or dissolution of salt crusts with addition of fresh pond waters. The Hardie-Eugster Model for evaporative salinization of waters (fig. 2) begins of calcium and alkalinity. with pond water containing the major positively and negatively charged ions fate minerals of magnesium such as epso- the last minerals to dissolve were the first (cations and anions). As evaporation takes mite, hexahydrite, and starkeyite, sulfate to precipitate out (silicates, carbonates, place, one of the first minerals in large minerals of sodium such as mirabilite and etc.). quantities to settle out is calcite. After thenardite, double sulfate minerals of so- This cycle of evaporative salinization that. depending on the relative concentra- dium and magnesium such as loewite, and dilution-dissolution regulates the tions of calcium and alkalinity (COi-+ bloedite, and konyaite as well as halite chemistry and salinity of pond waters. It HCOJ, either gypsum or sepiolite would (NaCl). All of these evaporite minerals also affects hydraulic conductivities of precipitate, followed by other chemical have been identified in salt crusts in dry pond bed materials, and influences any changes. The resulting waters would be creek beds in the Upper Colorado River considerations of drain water reuse by typical of those in, for example, Death Basin during intensive studies by Tanji blending with fresh water or direct reuse. Valley and the Carson Sink, or the world’s and others. The Mancos Shale, a primary oceans and the Salton Sea, or Owens and source of dissolved mineral salts in this Pond design Pyramid Lakes. basin, is geochemically similar to the Current design criteria for saline-wa- Since the chemistry of subsurface Moreno Formation in the California Coast ter evaporation ponds rely heavily on US. drain waters in the west side of the San Range, a source of salts and trace ele- Soil Conservation Service specifications. Joaquin Valley is dominated by sodium, ments that may find their way into evapo- The pond must be large enough to satisfy calcium, and sulfate, along with some ration ponds. needs of the land area being drained, the chloride and bicarbonate, it is expected Following evaporative salinization of volume of subsurface drain water collect- that brine waters produced in evaporation drain waters in ponds, the deposited salt ed, and the rate of evaporation for that ponds from this region would be similar to crusts would be subject to dilution-disso- region. Ponds must have: a minimum em- ocean water. lution, that is, mineral dissolution from bankment top-width of 14 feet; freeboard Tanji and Doneen in 1966 tested a the introduction of fresh subsurface drain of 1.6 feet or equal to the maximum wave chemical model on evaporative saliniza- waters into the ponds. The sequence of runup; an inside slope of 6:l and outside tion of bicarbonate waters and soil solu- mineral dissolution is just the opposite of slope of 2:l; and a foundation stripped of tions and predicted the pH and precipita- evaporative salinization. The first miner- all vegetation. Seepage through the em- tion of calcite quantitatively. More als to dissolve were the last to precipitate bankment and bed material must be con- recently, Greg Smith, a graduate student out (chloride and sulfate minerals), and trolled, and there must be a subsurface in water science at UC Davis, combined the 1966 Tanji-Doneen model for calcite TABLE 1. Precipitation of calcite (CaC03) and gypsum (CaS04 2H20) in Panoche Drain Water precipitation with the 1969 Tanji model undergoing evaporative concentration for solubility of gypsum. Smith’s comput- Evaporative concentration factor er-calculated data for a Panoche Drain Panoche Drain Water 1x 2x 1ox water sample undergoing two-and ten------mi//imo/es//iter ------fold concentration in volume from evapo- Calcium (Ca2+) 20.0 5.7 1.6 ration show that substantial quantities of Sodium (Na+) 24.5 49.0 245.0 calcite and gypsum are precipitated out Magnesium (Mg2+) 13.0 26.0 130.0 (table 1). Chloride (Cl-) 15.6 31.3 156.0 Sulfate (SO%-) 43.0 54.0 245.0 After calcite and gypsum have settled Bicarbonate (HC03) 3.0 1.4 2.8 out of the pond water, the sequence of Carbonate (Cog-) 0 0.04 0.2 mineral precipitation in brines becomes DH 8.3 8.4 8.6 complex. Other carbonate minerals of so- Calcite precipitation _. 2.3 13.6 dium may precipitate, such as nahcolite, Gypsum precipitation - 27.1 180.0 soda, and trona, followed by hydrated sul- Source: Greg Smith, Department of Land, Air, and Water Resources, University of California, Davis. 1985.

CALIFORNIA AGRICULTURE, SEPTEMBER-OCTOBER 1985 11 conductivity is about 0.01 foot per year when the pond overlies usable groundwaters or about 1 foot per year when the pond overlies unusable groundwaters.) Another advantage is that the design may be a cost-effective alternative to double- lining, if that is required. The effective- ness of double-lining ponds and pits containing hazardous substances is currently being debated. Regulatory policy The interim waiver policy established by the California Regional Water Quality Control Board - Central Valley Region - allows certain agricultural subsurface drain-water disposal activities without Subsurface drainage evaporation ponds, which may take more than a quarter of the land formal adoption of state waste discharge being drained out of production, have to meet rigid state standards to minimize possible requirements. This policy specifies evapo- adverse environmental effects. Below, drainage water is pumped from a subsurface col- ration pond design and maintenance to lection system to pond surface. About 50 inches of saline water per year can be dis- minimize adverse environmental effects. posed of through pond evaporation. (Photos courtesy George True). For other Central Valley areas, the Regional Water Quality Control Board is- sues permits for evaporation ponds on a case-by-case basis. Agricultural drain waters generally are not considered to be a hazardous waste. But, if the constituents in the subsurface drainage waters exceed the limits specified in the California Ad- ministrative Code, drain water would be subject to the more stringent regulations governing surface impoundments. These regulations require that surface impoundments be double-lined, the cost of which may be too high for irrigated agriculture. A section of the Code does provide for exemptions from waste discharge requirements. The regional board may grant exemptions if it is found that hazardous waste constituents will not mi- grate from the surface holding area and pose a significant potential of polluting groundwater basins or surface waters. Summary In the absence of a master drainage system for the San Joaquin Valley and the increasing constraints placed on dis- Ground surface drain around the perimeter of the pond. charge of saline irrigation return flows, Water surface Internal dikes may be designed to con- the construction of on-farm and regional - struct cells within the pond and to allow evaporation ponds is one of the options for transfer of water from cell to cell and disposal of saline subsurface drain wa- deposition of salts in a progressive evapo- ters. It is essential that evaporation ponds ration system. be designed and operated in an environ- Innovative and cost-effective improve- mentally acceptable and efficient man- ments in pond design need to be evaluat- ner. Pond design must provide effective ed. For instance, Mark Grismer has pro- containment of salt and toxic constitu- posed a drainage system directly beneath ents, and information is needed on pond the pond instead of only the perimeter water chemistry and mineralogy so that (fig. 3), which would intercept nearly all potential reuse of drain waters and the seepage through the bed material and re- extent of chemical and biological immo- circulate it to the pond. One advantage of bilization of toxic constituents can be as- this design is that the interim waiver poli- sessed. Fig. 3. Proposed evaporation pond design cy requirement specifying the maximum in which drain tiles are placed beneath the Kenneith K. Tanji is Professor of Water Science. pond instead of at the perimeter. Schemat- soil-barrier hydraulic conductivity under Mark E. Grismer is Assistant Professor of Agricul- ic shows calculated flow lines, which would a pond could be relaxed (it is difficult to tural Engineering and Water Science, and Blaine R Hanson is Extension Irrigation and Drainage Spe- intercept seepage through bed material monitor the rate of seepage under ponds). cialist, Department of Land, Air and Water Re- and recirculate it to pond surface. (The maximum permissible hydraulic sources, University of California, Davis.

12 CALIFORNIA AGRICULTURE, SEPTEMBER-OCTOBER 1985