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SEQUENTIAL USE OF RECLAIMED WASTEWATER DESTINED FOR AQUIFER RECHARGE by Henry H. Smith Technical Report No. 4 June 1979 WATER RESOURCES RESEARCH CENTER Caribbean Research Institute College of the Virgin Islands St. Thomas. United States Virgin islands SEQUENTIAL USE OF RE1AINED WASTEWATER DESTINED FOR AQUIFER RECHARGE By Henry H. Smith Completion Report OWRT Project No. A—00l-VI Agreement No. 14-34-7100 September 1976.-September 1978 June 1979 The work upon which this report is based was supported in part by funds provided by the United States Department of the Interior, Office of Water Research and Technology as authorized under the Water Resources Act, of 1964., P.L. 88—379. Technical Report No. 4 Water Resources Research Center Caribbean Research Institute College of the Virgin Islands St. Thomas., USVI 00801 ABSTRACT Effluent from the advanced wastewater treatment plant in St. Croix, U.S. Virgin Islands, a water- deficient island, was used to recharge aquifers by use of spreading basins. Wells in the recharge area were monitored to determine changes in static levels and in quality. Results indicate an increase in the quantity of water in the aquifer and an upgrading in the quality of the eff1uent. An economic analysis showed the cost of recharge/recovery operations to be significantly less than the. Cost of desalination of sea water which supplies the major portion of water to the island. CONTENTS Page LISTS OF FIGURES vi LIST OF TABLES vii INTRODUCTION AND OVERVIEW 1 THE ADVANCED WASTEWATER - 3 TREATMENT PLANT HYDROGEOLOGICAL BACICG ROUND 3 PROCEDURE 4 RESULTS AND DISCUSSION 5 BIBLIOGRAPHY 24 LIST OF FIGURES Page I. Location of St. Croix, U.S. Virgin Islands 2 II. Project Location and facilities 6 III. Effect of Recharge Operations on Project Well PW-8 8 IV. Effect of Recharge Operations Project Well PW-9 9 V. Effect of Recharge Operations on Control Well A—18 10 VI. Effect of Recharge Operations on Control Well GG-7 11 VII. Effect of Recharge Operations on Control well FP—2 12 VIII. Monthly rainfall data for Golden Grove and Kingshill in St. Croix from September 1975 to February 1978 14 IX. Comparison of Chloride Levels in Project Wells and Treated Effluent 16 X. Comparison of Nitrate Levels in Project Wells and Treated Effluent 17 LIST OF TABLES Page I. ChJoide Levels Observed in Reclamatjon Project 15 II. Hardness Levels Observed in Reclamation Project 18 III. Turbidity Levels Observed in Reclamation Project 19 IV. Conductivity Levels Observed in Reclamation Project 20 V. Nitrate Levels Observed in Reclamation Project 21 VI. Estimated CostS for the Production and Recovery of Reclaimed Wastewater by Ground Water Recharge 22 INTRODUCTION AND OVERVIEW The U.S. Virgin Islands are located about 1,400 miles southeast of New York, 1,100 miles east—southeast of Miami and about 50 miles east of Puerto Rico (figure I) The Virgin Islands are part of the Lesser Antilles of the West Indies which separate the Caribbean Sea from the Atlantic Ocean St. Croix, the island on which this study takes place, is the largest (about 84 square miles) and southernmost of the Virgin Islands. Potable water is not an abundant resource in St. Croix or any of the Virgin Islands. Average rainfall is about 44 inches per year, stream flow is mostly ephemeral and high temperatures along with constant trade winds cause about 90 percent of the rainfall to be lost to evapotranspiration Aquifers are small and limited. While water consumption on St. Croix has been increasing due to population and life style changes, water production has not kept pace. Desalting of seawater accounts for about 60 percent of the water supply, ground water provides about 20 percent, rainwater stored in cisterns accounts for about 13 percent and the remaining 7 percent is usually attributed to recycled water The high cost of desalting, about $15 per thousand gal1ons, and the increasing unreliability of the desalination plants have increased the importance of water conservation as well as the development of alternative sources of water. With development of an alternative source in mind,, this project was undertaken The objectives of this study as proposed originally were to determine to what degree the use of advanced tertiary treated wastewater for (a) culture of fish for food, (b) trickle irrigation of vegetables and row crops, (c) spreading from irrigation of grasslands or (d) other related purposes might affect both the quality and quantity of wastewater for recharge. Due to unforeseen circumstances, several of the proposed areas of study could not be addressed. However, some recharging was done and records were kept of the’ amount of ainfa1l and quantity of advanced treated water recharged and the quality of this water. Also monitored were several wells in the vicinity of the recharge area Records of Static water levels and several biological and chemical quality parameters of these wells were maintained Interpretation of the data collected should provide some indication as to the effectiveness ’aquifer recharge in St. Croix Figure I Location of St. Croix, 13.2. Virgin Islands (Buros: L976a) THE ADVANCED WASTEWATER TREATMENT PLANT The source of water for recharge was the 0.5 million gallon per day (mgd) Advanced Wastewater treatment Plant (AWWTP) at East Bethlehem Middle Works The AWWTP receives effluent diverted from the primary treatment plant It enters the aeration tanks at the AWWTP where colloidal and dissolved organic matter converts into larger microorganisms which can be mechanically removed Large surface turbines stzi and aerate the water to permit the microorganisms to metabolize the organic matter. In the clarifier the mixture of microorganisms from the aeration tanks is separated by gravity The clear water flows to the solids contact tank where chemicals are added to the water to produce a flow which removes remaining particulate matter by increasing their size so that they settle out or are caught on the multi—media sand anthracite filters, which filter the water after it leaves the solids contact tank After the sand—anthracite filter, the water enters the chlorine contact chamber and chlorine is added for disinfection before distributions to the recharge areas. HYDROGEOLOGIcAL BACKGROUND The treats.d effluent was used in recharging two geologically dissimilar areas Both of these areas, Negro Bay and Golden Grove, contain several wells, are relatively close to the areon government-owned land and are accessible for ready observation These similarities as well as their differences made them ideal study locations. In the Golden Grove area alluvial tnaterials overlay the Kingsbill Marl and fill an intermittent stream channel This alluvium has a maximum thickness of 70 feet. It consits predominantly of montmorillonitic clay of low permeability with generally three or four water—bearing lenses of sandy- gravel material, each having a maximum thickness of five feet. Though these aquifers appear to be continuous throughout reaches of the stream valley, they are not interconnected except by wells which penetrate two or more of the lenses. The piçzometric surface shows the water to be confized in the aquifers ad a free water table does not exist, The ground surf ace n the Golden Grove recharge area is about 50 feet above sea level while the aquifers range between 35 feet above sea level to approximately sea level. The Kingshill Marl in the Negro Bay area consists of coral debris and beds of sandy clay alternating with nearly pure limestone. In the recharge area the thickness of the marl is about 160 feet and the ground water level is about .80 feet below the surface. Solution channels are common and probably afford communication, and infiltration throughout most of the thickness of the marl. However, at a depth of 10 feet below the surface, a dense and apparently impermeable bed occurs at the recharge site. Above this dense layer, the marl is permeable and capable of a high rate of infiltration. These conditions suggest that water will move into the upper 10 feet of the formation for storage and future recovery. Because of the depth to the permanent water table and the nature of the formation, no direct mixing of the wastewater and ground water was envisioned. A yearly average of 44 inches of rain falls on St. Croix of which an estimated 90 percent is lost to evapotranspiration, about 6 percent becomes runoff and 4 percent recharges the ground water. Since the aquifer at Negro Bay is exposed at the surface, rainfall was expected to have some effect on direct recharge. The Golden Grove site is within a broad alluvial valley where infiltration from an area of some six square miles upstream from the recharge site contributes to the ground water source. PROCEDURE Spreading basins and spray irrigation were used in the recharge operations. In Golden Grove six basins, each having a bottom area of 10,000 square feet, were used on a rotating basis in order to maintain a wet-dry cycle. The basins were excavated earthen ponds planted with Bermuda grass to promote bank stabilization, nutrient removal and percolation. Throughout the project, a water depth of approximately3.5 feet was maintained to permit natural surface aeration 1’hile minimizing evaporation losses and providing adequate head for rapid infiltration and percolation. Periodically, accumulated silt was scraped from the basin surface to maintain good infiltration rates. In the Negro Bay recharge area, two smaller spreading basins were used. Each basin had a bottom area of approximately 2,500 square feet. In addition to spreading basins, water was also recharged at Negro Bay by spray irrigation . The area was approximately 80,000 square feet. Recharging operations began in February 1974. Both quantity of water recharged and rainfall at the sites were monitored. Monthly records were also kept of the static well levels and several quality parameters of the effluent as well as the well water Wells used to monitor the effects of the recharge operations were located in the recharge areas while several wells in the vicinity not expected to be affected by recharging were monitored to serve as controls The locations of the recharge areas and wells monitored as shown in Figure II. RESULTS AND DISCUSSION The recharge facility was initially in operation from February 1974 until October 1974 In June 1974 operations were interrupted due to failure o one of the effluent pumps. In October the Frederiksted service area was connected to the treatment plant. This resulted in recharge operations being halted because the town of with its saltwater flush and fire fighting system, introduced about 0.08 mgd of saltwater causing a significant rise in the total dissolved solids (TDS) in the treated wastewater Prior to this, the treatment facility serviced only the mid-island area of St Croix By March 1976, the saltwater system in Frederiksted was converted to potable water and recharge operations continued In April 1976, the effluent force main was broken but recharge operations resumed in May. In August 1977 the Christiansted collection system was completed and connected to the treatment facility. Again, high chlorides in the effluent necessitated the suspension of the project. Artificial recharging in the Negro Bay area was discontinued after August 1974 because the soil is unfvorab1e ‘for recharge basin structures During dry periods almost all of the water applied was lost to evapotranspiration and there was no buildup in the groundwater. Most recently a splitter bog, designed to separate the Christiansted waste steam with itS high chloride concentration from the other waste streams with an acceptable level of chloride, was installed at the reclamation plant. It is expected that recharging will resume shortly. Figure II Project Location and facilities (Buros: 1976b) An estimate of the amount of recharge water reaching the ground water during the 16-month period from March 1976 to July 1977 may be made. During this period 53.31 million gallons of effluent were applied to the recharge areas. Since the six spreading basins were used on a rotating basis, we will assume that at any given time half of the basins (30,000 sq. ft.) were covered with water. To determine the depth of water applied over the 16-month period it is first necessary to convert the total water applied to cubic feet. 53,310,000 gals. x 0.134 cu. ft./gal. = 7,143,540 cu. ft.. The depth of water over the area then is: 7,14.3,540 cu. ft. 30,000 sq. ft. = 238.12 ft. The rainfall at the area must be considered. For this very rough estimate, the Virgin Islands average rainfall of 44 inches per year will be used. For a 16-month period this amounts to 4.89 ft. Depth of water added to the area then becomes: 238.12 ft. + 4.89 ft. = 243.01 ft. According to Bowden (1968) the annual pan evaporation in the Virgin Islands is 70 inches (5.83 feet) or 7.78 feet in 16— months. This amount lost must be subtracted from the depth of water added; 243.01 ft. — 7. 78 ft: = 235.23 ft. Over the recharge area of 30,000 square feet this amounts to 7,056,900 cubic feet or 52,785,612 gallons. In the 16—month period during which 53.31 million gallons of water was recharged, assuming normal rainfall and evaporation, approximately 52.79 million gallons of water were added to the ground water in the recharge area. Figures III - VII illustrate the effects of the recharge operations on the static well levels for 1976 and 1977. From the map of the site, Figure II, it can be seen that well PW-8 and PW-9 are located in the Golden Grove recharge area and are expected to be influenced by the recharging while wells A-18, GG-7 and FP-2 hou1d not be affected Well A-18 is ot 3,500 feet above the recharge area while wells GG-7 an FP-2 are below the recharge area. The curves of the static well levels of the control wells (A-18, GG-7 and F? 2) approximate the same shape showing the influence of rainfall during the project period Rainfall for 1976 and 1977 is plotted in Figure VIII. It is worthwhile to look in detail at two of the most significant quality parameters. Figure IX shows a comparison of chloride levels in the treated effluent and in samples obtained from the project wells PW—l and PW-4. Table I lists these levels as well as those for the control wells A-16 and FP—8. The mean level of chlorides observed in the treated effluent was 417 mg/1 while in PW-l and PW-4 the mean levels were 285 and 281 mg/i respectively. Unfortunately, data was not collected prior to the start of the recharging operations so it is not possible to say definitely if and to what degree recharging of treated effluent raised or lowered chloride levels in the project wells. The control wells, A-16 and FP 8, showed average chloride of 163 and 582 mg/1 respectively. This at least permits the conclusion to be made that the chloride levels in the recharge area are not higher or lower than the chloride levels of wells not in the recharge area. Another interesting parameter to examine is nitrates. While the effluent had an average nitrate level of 17.5 mg/1, the average levels of nitrate in wells PW-l and PW-8 were significantly lower at 3.8 and 2.6 mg/1 respectively. This occurrence, reduction in nitrate levels of recharge water, has been observed elsewhere (Clark, l977:765) and has been attributed to denitrification mainly by the vegetative covet of the ground surface. Figure X graphs the nitrate levels in the effluent and in the observation wells PW-1 and ?W-4. Table II through Table V show the characteristics of the effluent used for recharging, the water of the three wells in the project area (PW-l, ?W-4 and PW- 3) as well as the water of control wells A-16, FP-8 and GG-8. In many instances the water obtained from the project wells was of better quality than the water used for recharging or water form the control wells. The costs, associated with the production and recovery •of reclaimed water by ground water recharge are listed in Table VI. The total cost (production and recovery) of reclaimed FIGURE VIII TABLE 1 Chloride Levels Observed in Reclamation Project Sept. 1976 326 300 300 160 610 Oct. 390 310 270 150 560 Nov. 327 300 280 160 310 Dec. 365 300 290 Jan. 1977 366 320 310 Feb. 396 270 270 185 480 Mar. 426 300 305 600 April 464 234 301 701 May 300 280 June 258 300 665 July 531 290 305 650 Aug. 320 300 175 Sept. 570 270 300 115 570 Oct. 428 212 293 172 550 Nov. Dec. Jan. 1978 300 180 650 Feb. 210 160 670 March April 230 170 545 Table II Hardness Levels observed in Reclamation Project Sept. 1976 280 372 268 456 Oct. 456 360 268 384 Nov. 144 416 356 292 324 Dec. 412 380 Jan. 1977 148 392 356 Feb. 154 398 372 284 352 Mar. 190 460 360 430 April 194 384 388 424 May 388 360 June 394 385 416 July 297 452 380 Aug. 384 372 260 Sept. 416 384 222 Oct. 384 380 268 384 Nov. Dec. Jan. 1978 376 388 248 420 Feb. 398 416 292 412 Mar. April 394 276 400 TABLE III Turbidity Levels observed in Reclamation Project Sept. 1976 0.29 0.16 0.13 0.17 0.18 Oct. 0.48 Nov. 0.46 0.42 0.26 2.6 0.57 Dec. 0.28 0.2 0.15 Jan. 1977 0.6 0.18 0.22 Feb. 0.55 0.23 0.39 0.44 0.41 Mar. 0.32 0.19 0.26 0.23 April 0.24 0.19 0.26 0.27 May 0.5 0.2 0.49 June 0.28 0.22 0.16 July 0.39 0.23 0.18 0.26 Aug. 0.25 0.19 0.36 0.22 Sept. 0.26 0.22 0.16 Oct. 0.31 0.43 0.42 Nov. Dec. Jan. 1978 0.19 0.38 2.4 0.19 Feb. 0.32 0.34 1.5 0.31 Mar. April 0.41 0.91 0.28 TABLE IV Conductivity Observed in Reclamation Project Sept. 1976 1700 1800 1300 2800 Oct. 1500 1600 1700 1300 2600 Nov. 1200 1550 1700 1300 1750 Dec. 1207 1550 1700 Jan. 1977 1190 1650 1750 Feb. 1314 1700 2620 1350 2450 Mar. 1710 1750 1880 3000 April 1460 1590 1700 2880 May 1800 1800 June 1600 1800 2890 July 1794 1750 1900 2990 Aug. 1675 1800 1360 2950 Sept. 2500 2220 1550 3400 Oct. 1700 2000 1600 3000 Nov. Dec. Jan. 1978 2700 3100 2350 5000 Feb. 2750 3000 2300 5500 March 1800 1400 2600 TABLE V Nitrate Levels Observed in Reclamation Project Sept. 1976 17 4.9 2.2 3.6 3.8 Oct. Nov. 16 8.6 2.2 3.4 2.4 Dec. 17 7.8 2.4 Jan. 1977 19 5.4 2.4 Feb. 23 10 14.7 15.4 Mar. 25 3.1 2.1 1.7 April 23 3.2 2.2 3.1 May 2.6 1.9 June 2.6 1.9 2.2 July 3.4 1.7 1.5 Aug. 1.3 1.5 1.5 2.9 Sept. 0.3 0.3 0.3 0.1 0.3 Oct. 3.5 2.5 1.8 1.8 Nov. Dec. Jan. 1978 2.5 1.8 0.2 4 Feb. 3.7 3.2 4 4.3 March April 3.1 3.9 TABLE VI Estimated Costs for the Production and Recovery of Reclaimed Wastewater by Ground Water Recharge PROCUDUCTION-ANNUAL COSTS I. Depreciation (20-YR, straight—line) Initial Cost $800,000 $ 40,000 Phase 1 Improvements 30,000 1,500 Total Depreciation $ 41,500 II. MAINTENANCE AND REPAIR 36,000 IlI. LABOR Project Director $ 22,400 $ 22,400 Plant Superintendent 16,800 16,800 Chief Operator 11,500 11,500 Operator 9,775 19,600 Operator Trainee 8,050 16,100 Chemist 9 13,440 13,400 Secretary 8,050 4,000 Labor Subtotal 103.800 15 Percent Fringe Benefits 15,570 Total Labor 119,370 TOTAL ANNUAL COST $196,870 PRODUCTION-UNIT COSTS ($/Thousand Gal.) The annual cost on a unit basis with 15 percent downtime $ 1.27 Coagulant—aluminum sulfate, 50 mg/lb @ $1.20/lb .084 Chlorine, 20 mg/i 8 $0.50/lb .084 Power .30 Total Production Costs $ 1.74 Recovery—Unit Costs ($/thousand gal.) If 85 percent of recharged water is recovered by wells 2.04 Cost of ground water recovery3 0.30 Total Cost-Production and recovery ($/thousand gal.) $ 2.34 1. Based on Buros, 197Gb, p. 121 2. Includes operation of the recharge facilities 3. Includes all costs of drilling operating the wells water at the AWWTP, as it is being operated now with a production capacity of 0.5 mgd, is estimated to be $2.34 per thousand gallons. This compares favorably with the price of desalted water ($15 per thousand gallons and cistern water $20 per thousand gallons) As production capacity of the treatment plant increases, costs will decrease. With a plant production capacity of 0.75 mgd, cost of reclaimed water is expected to. decrease to $2.10 per thousand gallons and with a production capacity of 1 mgd, cost will be $2.01 per thousand gallons. However, it must be kept in mind that as plant production increases the limiting factor becomes the capacity of the recharge areas. At some point the capacity of the aquifer will be exceeded and will not be able to accept additional treated effluent. The cost of reclaimed water will be lowered if processing beyond primary treatment becomes required at the waste treatment plant before discharge. Once secondary treatment is required, Buros (1976b) estimates that costs attributable to reuse will drop by. about 75 percent. Use of the treated effluent for intermediate purposes before it becomes ground water (sequential use) promises to reduce the cost of recharge/recovery operations even further. Some of these intermediate uses have been determined to be irrigation, clam, culture and pisciculture. Detailed investigation of these sequential uses were intended to be part of this study. Problems associated with power outages, equipment failure and insufficient wastewater available for treatment curtailed the extent of the study, yet what was done indicates that reclamation of wastewater is economically feasible even without sequential use. B I BLI OGRAPHY Bowden, M.J. et al., 1970. “Climate, Water Balance and Climate Change in the North-West Virgin Islands,” St. Thomas: Caribbean Research Institute. Buros, O.K., 1976a. “A Water Management Plan for St. Croix, U.S. Virgin Islands,” Gainesville, Florida: Black, Crow & Eidsnss, Inc. Burns, O.K.., 1976b. “Wastewater Reclamation Project, St. Croix, U.S. Virgin Islands,” Cincinnati: Environmental Protection Agency. Buros, O.K., 1976c. “Water Reuse arid Conservation in the Virgin Islands,” Desalination Vol. 23, pp. 485—494. Clark, John W.. Hartmer, Mark J.., and Viessman, Warren, 1977. Water Supply & Pollution Control, New York: Thomas Y. Crowell Company, Inc. Jordan, D.G., 1975. “A Survey of the Water Resources of St. Croix, Virgin Islands,TM U.S.G.S. Caribbean District open-file Report, San Juan. Puerto Rico. Metcalf & Eddy, Inc., 1972. Wastewater Engineering, New York: McGraw-Hill Book CQmpany.