What the data shows about Rio Grande Valley source water — and why it requires a different approach to water treatment than most of the country
McAllen, Edinburg, Harlingen, Laredo, and the cities between them share something besides geography: source water from the Rio Grande that is among the most mineral-laden, variable, and treatment-intensive in Texas. For businesses running water-dependent equipment or processes — manufacturing, food service, healthcare, industrial operations — understanding what's in your water matters more here than almost anywhere else in the state.
The data shows a consistent picture.
What's in Rio Grande Source Water
Most RGV cities pull their municipal supply directly from the Rio Grande, treat it at surface water treatment plants, and push it through distribution. The river is where the challenge starts.
The U.S. International Boundary and Water Commission monitors TDS at multiple stations along the lower Rio Grande. Near McAllen and the surrounding area, TDS during normal flow periods runs in the range of 600–1,600 mg/L. Spikes to the upper end occur during low-flow periods — typically summer and early fall, when the river is running slow and irrigation return flows make up a larger share of the volume. The federal secondary standard for treated drinking water is 500 mg/L.
Water above 500 mg/L TDS starts affecting taste. Above 1,000 mg/L, it affects process quality in food and beverage operations and accelerates scale formation in industrial equipment. Industrial systems designed around average U.S. municipal water — typically 150–350 mg/L TDS — operate in a very different chemical environment when supplied from Rio Grande source water.
The TDS spikes aren't random. During dry periods, irrigation return flows — water that has been applied to crops, absorbed dissolved minerals from the soil, and drained back into the river system — make up a significant share of the Rio Grande's flow by the time it reaches Hidalgo County. The IBWC has monitored this pattern over decades. Average TDS stays within acceptable ranges during most periods, but spikes during dry years are a recurring feature, not anomalies.
Hardness — The Day-to-Day Problem
Texas already has among the hardest water in the country. In the Rio Grande Valley, municipal supply regularly exceeds 15 GPG. Laredo measures approximately 12.5 GPG in treated water; many RGV systems run higher.
To put those numbers in context: scale begins forming noticeably on heat transfer surfaces once hardness climbs into the very hard range (typically above 10 GPG). At 15 GPG, an untreated commercial steam boiler, chiller, or heat exchanger will accumulate enough mineral scale within a year to measurably cut efficiency. A quarter-inch of calcium carbonate scale on a heat exchanger surface reduces heat transfer efficiency by roughly 40%.
The same calcium and magnesium that create hardness also create scaling risk inside RO systems. High hardness drives the Langelier Saturation Index (LSI) into positive territory — meaning the feed water has a natural tendency to deposit scale on membrane surfaces. Without the right antiscalant chemistry sized for your actual feed water analysis, a South Texas RO system will foul faster and require more frequent cleaning cycles than the manufacturer's literature suggests.
| City / System | Verified Key Findings | Source |
|---|---|---|
| McAllen (MPU) | TTHM 81.7 ppb — the highest reading in our database against the 80 ppb annual-average limit (no violation reported); cyanide 120 ppb; arsenic 3.5 ppb | MPU 2025 CCR |
| Edinburg | TTHM 48 ppb — the 2021 TTHM violation is resolved; arsenic 2 ppb | City of Edinburg 2023 CCR |
| Harlingen | TTHM 45.5 ppb with individual samples past 50; arsenic 3.9 ppb; selenium 5.2 ppb | HWWS 2025 CCR |
| Pharr | TTHM 38 ppb (samples to 42.5); arsenic not reported as detected | City of Pharr 2024 CCR |
| Mission (Sharyland WSC) | One of the few systems publishing minerals: TDS 737 mg/L, hardness 281 mg/L as CaCO3 | Sharyland WSC 2024 CCR |
| Mission (municipal) | TTHM samples ranged 26–80 ppb — the top of the range is the federal limit; selenium 8.1 ppb | City of Mission 2024 CCR |
| Mercedes | TDS 1250 mg/L (over the 1000 secondary standard), hardness 325, sulfate 389; lithium 56 ppb | City of Mercedes 2025 CCR |
| Alamo | TDS 1100 mg/L, hardness 396, sulfate 311 — brackish-range supply | City of Alamo 2022 report (newest published) |
| Brownsville (BPUB) | TDS 772 mg/L, hardness 358; PFOS 5.3 ppt average — above the finalized 4 ppt federal MCL that takes effect later this decade; part of the supply already comes from brackish RO desalination (SRWA) | BPUB 2025 CCR |
| Weslaco (Military Highway WSC) | TTHM annual averages of 65–71 ppb against the 80 ppb limit | MHWSC 2025 CCR |
| North Alamo WSC (eastern RGV) | Documented TTHM MCL violations in late 2025 — locational annual average of 84 ppb vs the 80 limit | NAWSC 2025 CCR |
| San Benito | TTHM 66 ppb with samples to 72.8 | City of San Benito 2025 CCR |
| Laredo | TTHMs reached 105 ppb in 2020, exceeding the 80 ppb federal MCL; hardness ~12.5 GPG | City of Laredo CCR / Hydroviv (third-party; no verified profile yet) |
A note on how to read this: every value except Laredo's comes from the named utility's own Consumer Confidence Report — the annual disclosure each water system must publish under the Safe Drinking Water Act — read directly from the published report, not from third-party databases. Report years differ because utilities publish on different schedules; each city link goes to the full verified profile with the source PDF. Where TDS and hardness are absent, the utility didn't publish them (they're non-mandatory secondary constituents). Two systems carry documented TTHM MCL exceedances: North Alamo WSC (2025) and Laredo (2020, third-party sourced). The rest met federal standards in their most recent reports, though the contaminant picture has practical implications for water treatment design regardless.
Update: The table above now draws from our verified per-city water profiles — each number read from the utility's own latest Consumer Confidence Report. The complete list of systems, including Donna, San Juan, La Feria and Los Fresnos, is in the local water data index — which now reaches beyond the Valley to El Paso, Phoenix, Tucson, Las Vegas, San Antonio and Midland, each verified the same way.
What This Means for Industrial and Commercial Operations
High TDS, high hardness, and variable source water quality combine to make standard RO system designs — built around U.S. average municipal water assumptions — underperform in South Texas. A few specific consequences:
Membrane selection. Standard low-pressure tap water elements are rated for feed water in the 200–500 mg/L TDS range. Post-treatment Rio Grande source water regularly runs higher. Brackish water elements — DuPont FilmTec's BW30 series, Hydranautics' ESPA series, Toray's TMH line — are designed for 500–2,000 mg/L feed TDS and are the appropriate starting specification for most RGV commercial and industrial systems.
Antiscalant dosing. At 15 GPG hardness combined with the sulfate levels common in South Texas source water, a system running without antiscalant is a system that will scale. The dosing rate and chemistry should be based on an actual site water analysis, not a generic formulation. The difference between a correct antiscalant program and a wrong one shows up in CIP frequency and membrane replacement intervals.
Recovery rate. Standard design targets of 75–80% recovery work well in lower-TDS water. At 700–900 mg/L feed TDS, pushing to 75% recovery concentrates the reject stream to 2,800–3,600 mg/L, putting you well into the scaling risk zone for calcium carbonate and calcium sulfate. Many South Texas industrial systems run at 65–70% recovery to keep the concentrate chemistry manageable without excessive antiscalant loading.
Biofouling pretreatment. IBWC monitoring data shows elevated bacterial counts at several lower Rio Grande stations, driven by non-point sources including storm water, agricultural runoff, and urban growth. Municipal treatment addresses this, but it's a reason to keep cartridge filtration and pre-filter maintenance on a consistent schedule — particularly for systems with longer distribution runs between the treatment plant and the point of use.
The Seasonal Variability Problem
One feature of South Texas water that doesn't show up in annual average tables is how much TDS swings between wet and dry periods. The IBWC data shows Rio Grande TDS near McAllen ranging from roughly 300 mg/L during high-flow periods to over 1,600 mg/L when the river is running slow. A system designed and calibrated during a wet spring may start underperforming the following August when source water TDS climbs by 50% or more.
This is worth building into your monitoring protocol. Seasonal re-normalization of performance data, and periodic feed water testing to catch chemistry shifts, matter more in a variable-source-water region like the RGV than in areas with more stable groundwater or reservoir supply.
Local Knowledge, Local Supply
National distributors can ship you a membrane element. They can't tell you whether it's sized for your actual feed water, or why your system is fouling faster than the spec sheet suggested, or which antiscalant chemistry fits Hidalgo County source water in September versus March.
Enviro Membranes is based in McAllen. We stock brackish water elements from DuPont FilmTec, Hydranautics, and Toray with same-day availability for common configurations. If you're commissioning a new system, troubleshooting an existing one, or want a second opinion on your current membrane specification against an actual water analysis, reach us at enviromembranes.com.
Data sources: U.S. International Boundary and Water Commission (IBWC) Lower Rio Grande Water Quality Monitoring, 2013–2017; per-utility Consumer Confidence Reports 2022–2025 as cited in the table (source PDFs linked from each city's water profile page); City of Laredo CCR via Hydroviv; TCEQ Surface Water Quality Standards.