4.1 Hydrologic Cycle & Water Sources

Hydrologic Cycle & Water Sources

Quick Answer: Drinking-water sources are products of the hydrologic cycle. Texas operators must classify their supply as groundwater, surface water, or groundwater under the direct influence of surface water (GWUDI), then apply the TCEQ treatment and monitoring expectations that match that classification.

Every public water system starts with a source. On the TCEQ Class C and Class B water exams, source knowledge is not trivia — it drives which barriers you need, which contaminants you watch, and how you explain treatment upsets. If you know how water moves through the environment, you can predict what arrives at the plant or wellhead.

Stages of the Hydrologic Cycle

Water continuously cycles among atmosphere, land, and subsurface storage:

Exam tip: infiltration feeds groundwater; runoff feeds surface water. Mixing of the two near shallow wells is why GWUDI exists as a regulatory category.

Groundwater vs Surface Water vs GWUDI

Groundwater

Groundwater occupies pores and fractures in aquifers. Typical Texas groundwater traits:

• Lower and more stable turbidity than rivers
• Cooler, more constant temperature
• Often higher dissolved minerals (hardness, iron, manganese, TDS)
• Lower microbial risk when the aquifer and well construction are sound
• Possible natural contaminants (arsenic, fluoride, radionuclides) depending on geology

Many Texas community systems rely on aquifers such as the Edwards, Carrizo-Wilcox, Gulf Coast, Ogallala, and Trinity. Quantity can still fail during multi-year drought even when quality looks good.

Surface Water

Surface water includes rivers, reservoirs, lakes, and canals. Typical traits:

• Highly variable turbidity after storms
• Higher pathogen and organic loads
• Temperature and algae swings by season
• Requires a full multi-barrier treatment train (coagulation → flocculation → sedimentation → filtration → disinfection) under TCEQ surface-water rules

Texas reservoirs also accumulate nutrients that fuel algal blooms, taste-and-odor compounds, and higher disinfectant-demand water.

GWUDI (Groundwater Under the Direct Influence of Surface Water)

GWUDI is groundwater that is significantly influenced by surface water — for example, shallow alluvial wells near a river, springs with rapid recharge after rain, or wells that show large, rapid shifts in turbidity, temperature, or coliform after storms.

Regulatory consequence: Once TCEQ classifies a source as GWUDI, it is treated like surface water for filtration and disinfection credit purposes. Do not assume "well = groundwater-only rules." Classification follows evidence, not the pump house label.

Source Water Protection

Treatment removes contaminants after they enter the system; source water protection keeps them out. TCEQ and EPA emphasize multiple barriers, and the first barrier is land-use and wellhead/watershed control.

Core protection practices:

1. Delineate the wellhead protection area or watershed contributing to the intake
2. Inventory potential contamination sources (septic systems, livestock, fuel tanks, ag chemicals, industrial sites)
3. Manage risks through ordinances, setbacks, spill response, and public education
4. Monitor sentinel parameters (nitrate, coliform, turbidity, VOC screens) that flag early contamination
5. Coordinate with county health, emergency response, and neighboring systems

For groundwater, sanitary seals, grout seals, locked wellhouses, and sloping drainage away from the wellhead are daily operator responsibilities — not just paperwork.

For surface water, intake siting, reservoir management, and upstream spill notification matter as much as jar tests.

Texas Source Challenges Operators Must Anticipate

Drought and Quantity Stress

Texas drought cycles shrink reservoirs, lower water tables, and force systems onto emergency interconnects or alternate wells. Lower lake levels can also concentrate dissolved solids and organics. Exam questions often pair drought with increased source monitoring and conservation / alternate-source planning, not with "stop treating."

Hardness

Much of Texas groundwater is hard (calcium and magnesium). Hardness itself is mainly an aesthetic/operational issue (scale, soap use), but it interacts with:

• Lime softening chemistry
• Corrosion control decisions
• Coagulant dose and alkalinity consumption

Know that hardness is common; do not confuse it with turbidity or with fecal contamination.

Natural Organic Matter (NOM)

Surface waters and some shallow groundwaters carry NOM that:

• Raises coagulant demand
• Forms disinfection byproduct (DBP) precursors when chlorine is applied
• Can cause color, taste, and odor complaints

After storms or turnover events, NOM and turbidity often spike together — a classic reason to re-run jar tests and watch chlorine demand.

Other Texas-Relevant Notes

• Coastal and some inland aquifers may show chloride / TDS intrusion under heavy pumping
• Oilfield and industrial areas raise hydrocarbon / brine spill risk near sources
• Flash floods can overwhelm intakes and short-circuit clarifiers downstream

TCEQ Exam Framing

When a question describes a source, classify it first:

• Deep, stable well with low turbidity → groundwater expectations
• River/reservoir → surface-water multi-barrier treatment
• Shallow well near a creek that muddies after rain → think GWUDI

Then match the problem to the barrier: protection for land-use threats, coagulation/filtration for particles and pathogens in surface water, and disinfection for microbial kill/inactivation across source types.

Operator Checklist

• Know your official source classification on the TCEQ approval / PWS files
• Track raw turbidity, temperature, pH, alkalinity, and odor daily (or per SOP)
• After major rain, increase monitoring frequency and walk the wellhead/intake
• Document source protection activities — they are part of professional practice, not optional extras
• During drought, log levels, pumping rates, and customer restrictions; quantity failures are still compliance failures if you cannot meet demand safely