2.7 Source Water Characteristics, Contaminant Categories & Watershed Protection Plans

Reading a source before you treat it

The WPI outline lists five source water characteristics an operator must evaluate: biological (bacteria, protozoa, viruses), chemical, physical, potential sources of source water contamination, and supply. That list is the exam's organizing frame, and it is also the correct operational frame, because each characteristic maps to a different barrier in the treatment train.

Biological characteristics

The Surface Water Treatment Rules require 3-log (99.9 percent) removal or inactivation of Giardia, 4-log (99.99 percent) of viruses, and, under LT2, 2-log removal of Cryptosporidium for filtered systems. The reason the rules speak of removal or inactivation is precisely this table: chlorine handles viruses and bacteria well and Cryptosporidium essentially not at all, so the protozoan requirement is met by physical removal plus, where needed, UV.

Chemical characteristics

• Inorganics with health-based MCLs: arsenic, nitrate and nitrite, fluoride, selenium, antimony, barium, cadmium, chromium, mercury, thallium, cyanide, and radionuclides including uranium, radium, and gross alpha.
• Aesthetic (secondary) constituents: iron, manganese, sulfate, chloride, total dissolved solids, zinc, silver, and color.
• Organics: volatile organic compounds from fuels and solvents, synthetic organics including pesticides and herbicides, and PFAS, which Colorado regulates and monitors and which are removed principally by granular activated carbon, ion exchange, or reverse osmosis.
• Total organic carbon (TOC). Natural organic matter is the precursor that reacts with chlorine to form trihalomethanes and haloacetic acids. Raw-water TOC and alkalinity together set the enhanced-coagulation TOC removal percentage a conventional plant must meet.
• Alkalinity, hardness, pH, and temperature. These are not contaminants but they control coagulation chemistry, disinfection kinetics, corrosion, and softening decisions.

Physical characteristics

Turbidity is the master physical parameter: it interferes with disinfection by shielding organisms, indicates filter performance, and is itself a regulated treatment technique standard. Temperature governs reaction rates, dissolved oxygen, and CT — cold Colorado source water requires substantially more contact time for the same log inactivation. Color, taste and odor, and suspended and dissolved solids round out the group.

Where contamination comes from

Potential contaminant sources are conventionally split two ways:

• Point sources — a discrete conveyance: a permitted outfall, a sewer overflow, a leaking underground storage tank, a mine adit, or a chemical spill at a road crossing.
• Nonpoint sources — diffuse runoff: agriculture and grazing, urban stormwater, forestry and logging roads, failing septic systems, atmospheric deposition, and abandoned mine drainage. Colorado adds two that matter locally: wildfire burn scars, which deliver enormous turbidity, ash, nutrient, and manganese loads for years after a fire, and spring snowmelt, which concentrates a year's watershed loading into a few weeks.

Reservoir behavior: stratification and turnover

A Colorado storage reservoir is not a uniform body of water. In summer it stratifies into three layers:

• Epilimnion — warm, well mixed, oxygenated surface layer; algae grow here.
• Metalimnion (thermocline) — the zone of rapid temperature change that blocks vertical mixing.
• Hypolimnion — cold, dense bottom layer that becomes anoxic as settled organic matter decays.

Under anoxic conditions the sediments release soluble iron and manganese, hydrogen sulfide, ammonia, and phosphorus, and the water becomes reducing. In autumn the surface cools until its density matches the bottom layer and the reservoir turns over, mixing all of that upward within days. Spring turnover repeats the process after ice-out. The operational consequence is abrupt: raw manganese may jump from below detection to several tenths of a milligram per liter, taste and odor complaints spike, chlorine demand rises, and coagulant dose must be reset. Multi-level intake structures exist so the operator can draw from whichever depth has the best water — and knowing when to switch ports is a genuine operator skill.

Source water protection as a treatment barrier

Colorado's Source Water Assessment and Protection (SWAP) program, run by CDPHE's Water Quality Control Division, delineated a source water assessment area for every public water system and inventoried the potential contaminant sources within it. A system then develops a voluntary Source Water Protection Plan, usually with help from the Colorado Rural Water Association, containing:

1. Delineation of the protection area — a watershed boundary for surface sources, or a time-of-travel capture zone for wells.
2. A contaminant source inventory kept current.
3. Management approaches — land-use agreements and easements, spill response coordination with local emergency responders, road salt and hazardous materials routing agreements, septic system programs, grazing and forestry best management practices, and post-fire mitigation partnerships.
4. Contingency planning for the loss of a source, including interconnections and alternate supplies.
5. Public education and stakeholder involvement.

Following the plan is a listed WPI job task, and it is the outermost ring of the multiple-barrier approach: source protection, then treatment, then disinfection, then a protected distribution system, then monitoring. Every barrier you keep intact reduces what the next one has to do.