Topic: Saline-Water Zone/ASR
Source: Carollo Engineers
The Barton Springs/Edwards Aquifer Conservation District (BSEACD or the District) was formed to conserve, protect, and enhance the groundwater resources in its jurisdictional area, which covers the unconfined (recharge) zone and the confined zone of the Barton Springs segment of the Edwards Aquifer in central Texas.
The Edwards Aquifer has been considered a vast source of inexpensive, high-quality drinking water for many years. However, restrictions have been placed on production from the Edwards in recent years, and rising demands have increased faster than the provision of other additional sources. With the past significant reliance on the Edwards Aquifer, other potential sources warrant further consideration. Potential sources within the boundaries of the BSEACD that are being minimally used, if at all, include the Middle and Lower Trinity aquifers, and the brackish portion of the Edwards. One prospective new water supply source is the large quantity of brackish groundwater in the eastern portion of the District. Texas Disposal Systems is located on this “donut hole” which is outside the jurisdiction of the BSEACD. Multi-port wells installed here have provided data necessary to analyze the feasibility of desalination of the brackish groundwater; management of desalination treatment residuals; and using the treated water for aquifer storage and recovery (ASR).
An evaluation of desalination technologies to lower the total dissolved solids (TDS) of brackish Edwards Aquifer water to drinking water standards found reverse osmosis (RO) membranes to be the most effective option. A two-stage, single-pass RO system may be able to provide water meeting regulatory standards. However, the groundwater contains significant concentrations of boron. Boron does not have a primary or secondary maximum contaminant level, but is known to have negative impacts on plant life, depending on the concentration and plants involved. To remove boron, a second-pass, two-stage RO system would be able to reduce boron levels and help to provide high quality water that may be used for both irrigation and human consumption. For purposes of this feasibility assessment, the desalination facility was sized for 2.5 million gallons per day (mgd) in Phase 1 and 5 mgd in Phase 2, based on estimated yields from three potential brackish groundwater supply wells from the lower producing intervals of the Edwards Aquifer. Generating power from the Texas Disposal Systems landfill gas could meet the energy requirements of the Phase 1 and Phase 2 desalination facility as well as provide additional electricity. Several disposal options for the brine concentrate that is a byproduct of RO were evaluated, and deep well injection into the Trinity Aquifer at the Texas Disposal Systems site was selected as the most cost effective option.
Aquifer storage and recovery (ASR) is the storage of water underground in an aquifer and subsequent recovery of the stored water when needed. For the BSEACD, ASR could create new water supplies in central Texas, meet seasonal variations in water supply and demand, and enhance water supply reliability during droughts. This would be for the District’s permittees and potentially for other water users in the surrounding area. A portion of the produced desalinated drinking water would be stored during winter months when demands are low. During summer peak demand months and droughts, the stored water would be recovered from the ASR wells and added to the desalination supply, helping to meet peak demands exceeding the capacity of the desalination treatment plant. ASR wells would be located within the TDS “donut hole” area, storing water in the upper producing intervals of the Edwards Aquifer, which is brackish at this location.
Feasibility study level capital and operation & maintenance (O&M) costs for the desalination system, wellfield collection system, various concentrate disposal alternatives, an aquifer storage and recovery (ASR) system, as well as a landfill gas combined heat and power facility were developed for two distinct phases: 1) a desalination facility with a production capacity of 2.5 mgd; 2) a desalination facility with a production capacity of 5.0 mgd. These costs were used to develop five financial forecast scenarios, exclusive of ASR. The lowest cost of the evaluated scenarios is $6.51 per 1,000 gallons of production in the first year of operation for a 5 mgd desalination facility with a landfill gas to energy cogeneration facility and concentrate disposal in Trinity Aquifer injection wells. The 30-year life cycle cost for a 5 mgd desalination facility powered by traditional grid sources with concentrate disposal in Trinity Aquifer injection wells is $8.20 per 1,000 gallons. The 30-year cost of an ASR project for water produced by the 5 mgd desalination facility is $0.38 per 1,000 gallons.