3.2 Surface Water Sources, Intakes & Reservoir Management

3.2 Surface Water Sources, Intakes & Reservoir Management

The Need-to-Know Criteria require Water Treatment operators to "inspect surface water sources for issues that may affect water quality (e.g., non-native plant species, mussels, algae, erosion)," "perform raw water reservoir inspection, maintenance, and cleaning," and "manage stored water release based on forecasted demand." In Arizona these tasks center on a handful of very large, very hot, heavily managed reservoirs.

---

Arizona's Surface Water Sources

• Colorado River via the Central Arizona Project (CAP): a 336-mile canal delivering water to Phoenix and Tucson. Long open-channel residence time means warming, evaporation-driven mineral concentration, and algal growth in the canal itself.
• Salt and Verde Rivers: the Salt River Project reservoir chain (Roosevelt, Apache, Canyon, Saguaro on the Salt; Horseshoe and Bartlett on the Verde) serving the Phoenix metropolitan area.
• Local reservoirs and direct diversions serving smaller communities.

Common characteristics: high evaporation, elevated total dissolved solids, warm water for much of the year, extreme monsoon turbidity spikes, and long detention that favors algal growth.

---

Thermal Stratification: Why a Lake Is Three Lakes

Water reaches maximum density at about 39°F (4°C). As a reservoir warms through spring and summer, warm low-density water floats on cold dense water and the lake separates into layers that no longer mix.

The hypolimnion is the operator's problem. Cut off from the atmosphere and from photosynthesis, bacteria decomposing settled organic matter consume the oxygen. Once the water goes anoxic, reducing conditions convert insoluble ferric iron and manganic manganese into soluble ferrous iron (Fe²⁺) and manganous manganese (Mn²⁺), which dissolve out of the sediments. Sulfate reduction generates hydrogen sulfide with its rotten-egg odor, and ammonia accumulates.

Turnover

In fall, surface water cools until it matches the density of the deeper water, and wind then mixes the entire reservoir top to bottom. Fall turnover distributes accumulated hypolimnetic iron, manganese, sulfide, ammonia, and organic matter throughout the water column, including past your intake. In many climates a second, weaker spring turnover follows ice-out.

[!WARNING] Turnover is the single most predictable raw water quality crisis of the year. Symptoms at the plant: sudden manganese in finished water producing black water complaints, a jump in chlorine demand from ammonia and sulfide, taste and odor complaints, and coagulation upset from a changed raw water character. Anticipate it — do not react to it.

---

Intakes and Depth Selection

A multi-level (multi-port) intake tower has gated openings at several elevations, letting the operator select withdrawal depth. This is the most powerful raw water quality tool available, and the NTK expects operators to understand it.

Selection logic through the year:

• Summer stratified conditions: avoid the epilimnion during a heavy algal bloom (algae, high pH, taste and odor) and avoid the anoxic hypolimnion (iron, manganese, sulfide). Draw from the metalimnion.
• During turnover: the reservoir is mixed, so depth selection buys little. This is when treatment adjustments carry the load.
• High turbidity after monsoon runoff: draw deeper, below the turbid inflow plume, or above a density current running along the bottom.

Other intake features:

• Trash racks exclude logs and large debris; traveling screens or bar screens remove smaller material.
• Low-lift (raw water) pumps move water to the plant.
• Zebra and quagga mussel control — chlorination, copper-based coatings, or mechanical removal.
• Intake depth alarms protect against pumping air as reservoir levels fall.

Reservoir Inspection and Maintenance Tasks

Routine work includes checking for shoreline erosion and sedimentation reducing storage, non-native and invasive plant species, evidence of animal or human contamination in the watershed, algal bloom development, and the physical integrity of intake structures, gates, screens, and hoists. Sediment surveys quantify lost capacity over decades.

Managing Stored Water Release

The NTK asks operators to "manage stored water release based on forecasted demand." Practically, that means coordinating reservoir withdrawals against projected system demand, downstream delivery obligations, water rights and allocations, seasonal peaking, and the raw water quality expected at the chosen depth. In Arizona this is bound up with CAP allocations and Colorado River shortage tiers, which can change how much surface water a utility may take in a given year.

Algae, Taste and Odor, and Cyanotoxins

Warm, nutrient-rich, long-detention Arizona reservoirs and canals are excellent algae habitat.

Taste and Odor Compounds

The two dominant culprits are geosmin and 2-methylisoborneol (MIB), produced by cyanobacteria (blue-green algae) and actinomycetes.

• Geosmin produces an earthy, beet-like odor. MIB produces a musty, camphor-like odor.
• Human detection thresholds are around 5 to 10 nanograms per liter — parts per trillion. Customers will complain at concentrations far below any health concern.
• Neither is removed by conventional coagulation, sedimentation, and filtration, and free chlorine does not destroy them. Chlorinating a taste and odor event can make it worse by lysing algal cells and releasing more compound.

Effective controls: powdered activated carbon (PAC) dosed ahead of treatment, granular activated carbon contactors, ozone, or advanced oxidation. Source control includes reservoir aeration or destratification, and algaecide application, usually copper sulfate.

[!NOTE] Copper sulfate application is a judgment call. It kills algae, but cell lysis releases intracellular taste and odor compounds and, for toxin-producing cyanobacteria, intracellular toxins. Treating a heavy cyanobacterial bloom with an algaecide immediately upstream of an intake can create a worse problem than it solves.

Cyanotoxins

Harmful algal blooms of Microcystis, Anabaena (Dolichospermum), and Cylindrospermopsis can produce microcystins, cylindrospermopsin, anatoxin-a, and saxitoxins. EPA has issued health advisories for microcystins and cylindrospermopsin and lists cyanotoxins for monitoring consideration. Operating principle: keep cells intact through coagulation, sedimentation, and filtration so that toxins remain inside the cells and are physically removed, rather than lysing cells and releasing dissolved toxin that then requires oxidation or activated carbon.

Invasive Species

Quagga mussels are established in the Colorado River system and therefore in CAP-delivered water. They colonize intake structures, screens, trash racks, and raw water piping, restricting flow, fouling instruments, and creating taste and odor and corrosion problems when they die and decay. Control combines continuous or periodic chlorination at the intake, antifouling coatings, mechanical removal, and vigilance at boat launches. Non-native plant species such as salt cedar (tamarisk) along reservoir margins consume large volumes of water and alter shoreline conditions, which is why the NTK lists them among the inspection items.

Monsoon Turbidity

Arizona's summer monsoon delivers intense, short-duration storms onto sparsely vegetated watersheds, generating flash runoff that can raise raw water turbidity from a few nephelometric turbidity units to hundreds or thousands within hours. Operators respond by adjusting the intake depth, sharply increasing coagulant dose (confirmed by jar testing), adding a coagulant aid polymer, reducing plant flow rate to preserve settling and filtration performance, and shortening filter runs. Watershed erosion, wildfire burn scars, and post-fire debris flows all make these events worse and are legitimate inspection concerns.