The Paso del Norte region has grown from less than 50,000 people in 1900 to 2,000,000  today.  This is a twenty fold increase in population. The surface water supply, the Rio Grande/Rio Bravo, is renewable and relatively constant.  Our ground water resources, the Hueco and Mesilla Bolsons, are not easily recharged and have been declining and becoming saline at a rapid rate for the last forty years.  The El Paso and Las Cruces Water Utilities  have relatively well defined plans to meet water needs for the next twenty years.  Ciudad Juarez does not.  The purpose of this part of the overall study was to examine the quality and quantity of the water resources envisioned as future supplies and evaluate the impact that both emerging and existing water treatment technology will have on their utilization. This is being done using existing water resource reports and information on water treatment technology from technical journals.

Background:

        In order to understand El Paso del Norte Region’s water utilization, a review of all available project material (i.e. proposals, water utilities reports on future water use and potential resources, and papers) was conducted.  The water utilities in El Paso, Ciudad Juarez and Las Cruces were contacted and interviews conducted.  El Paso Water Utility had the most information available and was willing to discuss it in detail.  The Junta de Muncipal del Agua in Ciudad Juarez did not have a comprehensive plan available for review.

        Currently all of Juarez’s municipal and industrial (M&I) water comes from the Hueco Bolson.  Juarez plans to begin pumping the Mesilla Bolson in the near future once a delivery pipeline is complete.  Additionally, consideration is begin given to the possibility of treating Mexico’s 60,000 af Rio Grande allocation for M&I use and then providing the irrigators with treated wastewater in exchange.

        Las Cruces currently pumps all of its M&I water from the Mesilla Bolson.  Las Cruces is considering using Rio Grande surface water when the Mesilla does not meet the needs for future growth. This water would have to be transferred from agricultural use to M&I use.  A multi-purpose, i.e., multi-city, surface water treatment facility is in the preliminary design stages.

        A number of surface water alternatives have been evaluated by the El Paso Water Utility (EPWU) based upon their feasibility, quantity, quality, and cost .  The EPWU plans to minimize the use of ground water resources and thus preserve the region's aquifers for drought period use.    EPWU’s potential surface water sources include: 1. increased use of Rio Grande water; 2. construction of regulating reservoirs; 3. utilization of surface inflows below Elephant Butte Dam; 4. increased water availability created by water conservation through canal lining; 5. utilization of unavoidable operational spills; and 6. utilization of return flows from the Mesilla Valley during the non-irrigation season.  Increasing usage of Rio Grande water is being vigorously pursued through the Texas/New Mexico Water Commission.  Planning for the multi-city water treatment facility mentioned above is part of this effort.

Surface Water:

        The most important of these sources is the Rio Grande.  The Rio Grande is regulated/constrained by a number of agencies.  The Rio Grande Compact Commission is responsible for interstate and international allocation of the water.  For practical purposes, the El Paso/Ciudad Juarez area receives its water from Elephant Butte Reservoir.   This water is divided between the El Paso County Water Improvement District (EPCWID), the Elephant Butte Irrigation District (EBID) and Mexico.   Table 1 provides data showing the approximate division of this water among these entities.  The division can vary significantly from these numbers during drought and high runoff years.  These numbers serve as a starting point for evaluating the surface water resources available for use. Most of this water has historically been utilized for crop irrigation.  Because the water has a higher economic value in municipal and industrial (M&I) use, ways are being found to transfer the water from agricultural to M&I uses.  Figure 1 shows the average monthly flows in the Rio Grande for 1980 through 1990 as the stream flows from Colorado to El Paso, Texas.  The large decrease in flow as water moves downstream is due to heavy irrigation use above the study area.  Figure 2 shows the specific conductivity for the same time period and stretch of river as Figure 1 and clearly illustrates the increasing concentration of dissolved solids as water moves downstream.  Figure 3 shows the hardness for the same time period and stretch of river.

        One of the major challenges faced by the El Paso Water Utility is the treatment of Rio Grande water.  The Rio Grande is highly variable in quality.  Surface water supplies are being more tightly regulated than in the past to protect public health.  Increased demand and stricter quality standards are leading to the development and application of new technologies.

        Since the Rio Grande is the only annually renewable water supply, El Paso and, in the future, other M&I water utilities will increase their use of this resource.  Treatment of this water is difficult because the water is highly variable in quality over the course of the year.  Temperature changes predictably with season.

Treatment Technology for Surface Water:

        Suspended solids and turbidity can vary dramatically in short periods depending on flow and upstream precipitation. Conventional treatment technologies such as coagulation, flocculation, settling and filtration can be designed to handle these variables in an effective manner.  Conventional technologies are not effective for removing dissolved contaminants, other than hardness, that are present in the Rio Grande.

        Dissolved contaminants include high concentrations of total dissolved solids (TDS) and total organic carbon (TOC).  TDS includes hardness and alkalinity and is often referred to by the more generic parameter - salinity.   Salinity increases during lower flow periods in the river because a larger portion of the flow tends to be irrigation return flow.  Only hardness is easily removed using conventional technology, i.e. lime-soda ash softening.  TOC and TDS are not easily controlled using conventional technologies.  Membrane technology (nanofiltration) is now being utilized to control hardness and TOC.   TOC control is a topic of great interest because control of organic compounds in surface water is one of the most important methods for controlling the formation of disinfection byproducts (DBPs).   This has been recognized by the USEPA and has been incorporated into the Information Collection Rule (ICR) with which all utilities must comply.

        Since the only annually renewable water supply is the Rio Grande, El Paso and, in the future, other M&I water utilities will increase their use of this river.  Unfortunately, the Rio Grande cannot meet all future M&I water needs. Table 2 provides an idea of the sustainable populations that could be served if the Rio Grande was the region’s sole water source.  During an average water year, the Rio Grande could provide municipal and industrial water to 2,985,000 people at a rate of 160 gallons per capita per day.  Our current population is 2,000,000 and average water use is approximately 128 gallons per capita per day.  The region’s aquifers/bolsons have enabled the region to grow to its current population.  Even if all water from the Rio Grande were used for M&I purposes and none for agriculture, there are limitations to the population the river can support.    Therefore, future scenarios should include utilization of the region’s aquifers even though the region’s population cannot be sustained indefinitely by groundwater resources.

        Ground water is currently the largest source of water for the region.  Ciudad Juarez and Las Cruces obtain all their water from aquifers and the EPWU obtains 60%.  In order to evaluate the usability of the aquifers, ground water quality and quantity have been analyzed to understand the condition of the Hueco, Mesilla, and other regional aquifers.

        The ground water quantity and quality for  the region were estimated using existing information.  The New Mexico Water Resources Research Institute (NMWRRI) was contacted at New Mexico State University.  The NMWRR and the Texas Water Development Board have prepared a preliminary draft of the Transboundary Aquifers of the El Paso/Ciudad Juarez/Las Cruces Region Report .  This report was reviewed to generate a better understanding of the intricate aquifer system of the region.  This data and data from the New Mexico State Engineer’s Office has been used to help establish an estimate of the total quantity and quality of the water in the region's aquifers. The aquifers are shown in Figure 4 entitled Aquifers of the Paso del Norte Region.  Tables 3 is a spreadsheet showing the format used to calculate the volumes and quality of water in the region’s aquifers. The method used involved determining the surface area of the aquifer and then calculating the volume using the geologic cross sections. These values are estimates of the extractable water and salinity of the water contained in these aquifers.

Ground Water for Juarez

        Information about future plans concerning water demands for Ciudad Juarez are essential to fully understand Mexico's impact on regional groundwater.  Mr. Francisco Nuñez of Junta Municipal de Aguas y Saneamiento was interviewed but could not speculate on any immediate plans aside from a desire to develop a well field in the Mesilla Bolson.
 
        Ciudad Juarez well information was obtained from previous studies.  Six representative wells were selected to analyze quantity and quality information.   Total Dissolved Solids (TDS) and Water Table Elevation vs. Time graphs were produced and revealed a significant TDS increase with respect to a decrease in water table elevation. Applying pumping rate trends from 1975 to 1995 to the representative wells, projections for TDS and water table elevation were made to the year 2020. Figure 5 is an example of one of the six representative wells.  It is evident that water quality, as defined by total dissolved solids, will continue to deteriorate if pumping continues at the current rate.

Brackish Ground Water

        One very significant source is the potential use of brackish ground water (water with a TDS greater than 1000 mg/l). Currently, membrane processes similar to reverse osmosis are rapidly evolving so that it is now feasible to consider brackish ground water as a potential future water source.  This source will open up very large potential supplies in several aquifers.  Both Juarez and El Paso have large brackish water sources as shown in Table 3 and illustrated in Figures 7 and 8. The technology is not without problems.  Uses must be found for the brine concentrate, which will comprise about 10 percent of the water, pumped.
 
        The region's ability to develop alternative sources is tied to its ability to generate water revenues.  Domestic water rates for El Paso and Ciudad Juarez are compared to provide an idea of the relative ability to generate revenue.  Although not directly comparable, water rates are much lower in Juarez than in El Paso.   Revenue generation from water sales in Cd. Juarez is likely to continue at a much lower rate than in El Paso.  However, the cost of developing additional water supplies is similar for both cities.  The economics of the situation imply that Ciudad Juarez is likely to have a much more difficult time developing its potential water supplies.

Surface and Ground Water Production Costs for El Paso, Texas:

        The existing resources for water production have been identified as the Hueco and Mesilla Bolsons for ground water, and the Rio Grande River for surface water.  It is believed that alternative or additional water resources are necessary in order to sustain the population of the region.  A major factor in the consideration of these resources is cost.  As such, historical costs for current processes were identified and then projections made using regression analyses.

        In order to fully understand the demands for water and trends in water production for the region, historical data was obtained from the El Paso Water Utilities / Public Service Board.  Although this data solely applies to the EPWU / PSB, production and consumption trends are considered to be indicative of trends for the region.
 
        First, the rate structure of the EPWU / PSB was evaluated.  Data from EPWU indicated that the average single family residence (consisting of a four-person household) had an average winter consumption (AWC) of 11.92 CCF (hundred cubic feet) (equal to 33.75 m3 or 8916.78 gallons).  The AWC is used to determine the increment quantity per given rate (applicable after 1990, when block rate structures were implemented).  Data in Table 4 was calculated to represent the billed rates for a single-family residence for 30.86 CCF of billed water (with an AWC = 11.92 CCF).  (The 30.86 CCF quantity was used to represent 192.37 gallons per day consumption per person for four people for a period of one month consisting of thirty days.)

        In order to take inflation into account, the consumer price index for all commodities was obtained for each year, and an average was taken such that the prices of all commodities were determined to have increased at an average rate of 5.067% per year.  Based upon this rate, the billed rate for 1967 was inflated by 5.067% per year in order to identify the correlation between inflation and the increases in rates of billed water.  Figure 6 shows a plot of both the annual billed cost per thousand gallons and the annually inflated costs per thousand gallons (where each year's cost is equal to the previous year's cost multiplied by 1.05067, beginning in 1967).  Trendlines (and corresponding trend equations and R-squared values indicating statistical level of confidence) were fitted to each set of data.  (Values corresponding to the calculations for these billed rates may be seen in Table 4.)  Inspection of Figure 6 shows that there has been no significant increase in current water production costs over time.

        Billed water rates are dependent upon the cost of production of both groundwater and surface water. The amount of available surface water (from the Rio Grande River) is dependent upon annual precipitation in the upper reaches of the basin.  As such, additional groundwater has been used to supplement the water supply in years of decreased rainfall.  An analysis of the historical cost of surface and groundwater production (in dollars per thousand gallons) was done.  Table 5 shows historical water production data from 1967 to 1996, including quantities of raw and finished water for both groundwater and surface water.  Until 1993, all surface water was produced at the EPWU Canal Street Water Treatment Plant.  In March of 1993, the EPWU Jonathan Rogers Water Treatment Plant was opened.  The annual productions for each of these plants are also given on Table 5.  The quantity of annual surface water produced has increased significantly since 1967.   Increased annual demands have been met with surface water.

        The estimates for cost of production per thousand gallons took the cost of operation and maintenance for each EPWU plant and divided it by the quantity of water produced at that plant for the given year.  These values may be seen in Table 6.  The cost of production of groundwater is significantly lower than that of surface water.  The unit cost of surface water production, however, not only increased due to inflation, but also was highly dependent upon the quantity of water available.  Fixed operations and maintenance costs tend to dominate unit cost of production.  These calculations represented actual costs for the given years.

        In order to objectively analyze the increase in production costs, it was necessary to take inflation into account.  The prices of all commodities were determined to have increased at an average rate of 5.067% per year (U.S. Buearu of the Census, 492), and costs were adjusted for inflation (using the formula P = (1/(1.05067n-1970)) where n is the year evaluated) so that all costs were given in equivalent 1970 dollars.  When inflation is taken into account as shown in Figures 9, 10, and 11 there is no significant increase in water production costs.

        Figure 9 shows the annual cost of water production per thousand gallons for ground water.  A trendline has been generated for each set of costs, both actual and inflation adjusted, in order to project future costs. As indicated by the trendlines, the cost of production for groundwater has increased faster than the rate of inflation

        Figures 10 shows the annual cost of water production of per thousand gallons for surface water, and Figure 11 specifically shows the cost of water production per thousand gallons for the Canal Street Plant. The inflation trendline indicates that the increases in production costs for surface water (Canal Street Plant) have been predominately due to inflation.  However, as noted above, projections for surface water costs (including the Canal Street Plant) are difficult to make due to the dependence upon the region's annual precipitation.  Figure 12 shows the annual cost of water production per thousand gallons for the J. Rogers W.T. Plant.  Since this plant has been in operation since 1993, the given data is not believed to be indicative of future cost; thus a trendline was not considered applicable.  (Whenever possible, the equation of the trendlines are given with the corresponding R-squared value indicating the statistical level of confidence.)

        Having identified the increasing cost in the production of groundwater, as well as the cost of the production of surface water, alternative processes for the available water were then evaluated.  Electrodialysis response, nanofiltration using spiral-wound membranes (with costs given including and excluding brine disposal), and multi-flash distillation were determined to be viable process alternatives.  Using cost estimations from Cost Estimates for Membrane Filtration and Conventional Treatment (Figures 13 and 14), a cost was projected for a 10-MGD (million gallons per day) facility (Figure 15) for the nanofiltration process which includes the cost of brine disposal.  Cost per thousand gallons for both operations and maintenance costs and total plant costs are illustrated in Figure 16 for both 1-MGD and 10-MGD facilities for all three processes.
 
Conclusions

• Remaining brackish groundwater resources are very large relative to remaining fresh groundwater, i.e., water with a TDS of less than 1000 mg/l.

• Membrane technology exists for the economical utilization of brackish groundwater.

• Technology for the utilization of the brine concentrate resulting from desalination is being developed.

• The cost of surface water treatment for the El Paso Water Utility has increased at approximately the same rate as inflation despite increasingly stringent treatment requirements.

• Membrane technology is likely to ultimately replace much of the physical-chemical water treatment technology currently used to treat surface water due to the need to remove dissolved organic carbon.  Treatment costs will increase but not dramatically.

Maria Mayela Quezada, an Environmental Engineering Graduate Research Assistant (GRA), and Linda Troncoso, an Environmental Undergraduate Research Assistant (UGRA), have collected and assisted in the evaluation of the water information presented herein.  Both work under the direction of  Dr. Turner and without their assistance this report would not have been possible.

References

Association of Metropolitan Sewerage Agencies Application, El Paso Water Utilities / Public Service Board.

Boyle Engineering Corporation, El Paso Water Resources Management Plan – Phase II Report, El Paso, TX. 1992, A-1-39.

Cliett, Tom.  “Groundwater Occurrences of the El Paso Area and its Related Geology.”  New Mexico Geological Society – Twentieth Field Conference.  Date unknown.

EPWU / PSB Planning and Development Department, Water Resources Report (El Paso 1996).

EPWU  / PSB Water Division.  Field and well data. 1994-1996.

El Paso Water Utilities / Public Service Board.  Annual Budget Reports. 1971-1994.

Gates, J.S., et. al. Availability of Fresh and Slightly Saline Groundwater in the Basins of Westernmost Texas:  U.S. Geological Survey: Texas Department of Water Resources Report 286. Texas Water Development Board, Austin, TX. 1980.

Junta Municipal de Aguas y Sanatimiento, Juarez, Chih. Mexico.  Field data. 1969-1994.

Kennedy, John, New Mexico State University Water Resources Research Institute.  Personal interviews. 19 Dec. 1996, and 8 March 1997.

McKinney, Bill. Director of Las Cruces Water Resources. Telephone interview.  16 May 1997.

NMSUWRRI, TWDB.  Transboundary Aquifers of the El Paso / Ciudad Juarez / Las Cruces Region. To be released 1997.

Nuñez, Francisco, Director of Junta Municipal de Aguas y Sanatimiento, Juarez, Chih., Mex.  Personal interview. 6 October 1996.

Orr, Brennan R., et. al. Water Resources of the Rincon and Mesilla Valleys and Adjacent Areas, NewMexico:  Technical Report 43.  New Mexico State Engineers.  Santa Fe, NM. 1981.  Plates 2,8.

Rebuck, Ernest.  Manager, El Paso Water Utilities / Public Service Board.  Personal interviews.

Rittmann, Douglas.  Water Systems Division Manager, El Paso Water Utilities / Public Service Board.  Personal interviews.  27 June 1997, and 11 July 1997.

Sperka, Roger.  Geologist, El Paso Water Utilities / Public Service Board.  Personal interviews.

Turner, C., et. al. Preliminary Research Study of a Water Desalination Water Treatment Technology Program Report No. 6 for the USBR (Contract No.1425-3-CR-81-19500) by CE and ME Departments, College of Engineering, University of Texas at El Paso, June 1995, p. 40.

U.S. Bureau of the Census.  Statistical Abstract of the United States: 1995. (115th ed.). Washington DC, 1995, p. 462.

U.S. Bureau of Reclamation, July 1980. Special Report on Elephant Butte Reservoir-Fort Quitman Project, New Mexico - Texas.

Weisner, Mark R., et. al. "Cost Estimates for Membrane Filtration and Conventional Treatment,” Journal of the AWWA, Dec. 1994, p. 39.