2.1 Raw Water Sources, High-Altitude Characteristics & Reservoir Quality

Raw Water Sources, High-Altitude Characteristics & Reservoir Quality

Water treatment in Colorado presents unique operational hurdles rooted in extreme elevation, alpine hydrology, and complex raw water source dynamics. A certified operator in responsible charge (ORC) must understand the fundamental physical, chemical, and biological distinctions among raw water classifications, as well as how high-altitude climatic conditions directly dictate treatment kinetics and process performance.

---

1. Regulatory Source Water Classifications

Under the Colorado Primary Drinking Water Regulations (5 CCR 1003-1, Regulation 11) and the federal Safe Drinking Water Act (SDWA), source waters are classified into three primary regulatory categories, each carrying specific monitoring, treatment, and multi-barrier compliance mandates.

Determining GWUDI Status

In Colorado's fractured granite, karst limestone, and shallow alluvial aquifers along mountain streams, wells and infiltration galleries often draw surface-influenced recharge. CDPHE Regulation 11 requires a Microscopic Particulate Analysis (MPA) using consensus method protocols when a well meets any of the following risk triggers:

1. Proximity: Located within 200 feet of a perennial surface water body or within shallow alluvial gravels without a confining geological layer.
2. Water Quality Correlation: Rapid, synchronous fluctuations in raw water temperature (>1.5°C over short intervals), turbidity spikes (>0.5 NTU following precipitation), or sudden conductivity drops during spring snowmelt.
3. Biological Indicators: Identification of surface-derived bio-indicators under polarized light microscopy, including diatoms, chlorophytes, rotifers, plant debris, insect parts, Giardia lamblia cysts, or Cryptosporidium oocysts.

Once declared GWUDI, the system has a strictly defined compliance schedule to install approved filtration (or demonstrate certified alternative bank filtration credit) and achieve mandatory Giardia/virus log-inactivation.

---

2. High-Altitude and Alpine Watershed Dynamics

Colorado water utilities draw significant raw water from mountain snowpack runoff (May through July freshet) originating in alpine and subalpine catchments (7,000 to over 11,000 feet above sea level). While these pristine headwaters boast low anthropogenic contamination, they create severe kinetic and chemical challenges for water treatment plants.

The Spring Runoff Profile

During peak snowmelt, high mountain streams experience rapid hydrological shifts:

• Near-Freezing Temperatures: Stream temperatures range from 0.5°C to 4.0°C.
• Extremely Low Total Dissolved Solids: TDS is often $<50\text{ mg/L}$, resulting in low ionic strength.
• Depressed Alkalinity: Natural buffering capacity drops to 10–30 mg/L as $\text{CaCO}_3$ because meltwater has minimal contact time with weathering bedrock minerals.
• High-Velocity Turbidity Surges: Flashy streamflows scour mountain channels, driving raw turbidity from baseline levels ($<1.0\text{ NTU}$) to sudden peaks exceeding $50–200\text{ NTU}$ laden with glacial flour, fine silts, and colloidal clays.

Cold-Water Reaction Kinetics and Viscosity

Water temperature exerts a profound thermodynamic effect on every unit process in a drinking water treatment facility:

$\mu_{T} = \mu_{20} \cdot e^{\frac{E_a}{R} \left( \frac{1}{T} - \frac{1}{293.15} \right)}$

1. Dynamic Viscosity ($\mu$): At 20°C, the dynamic viscosity of water is approximately 1.00 centipoise (cP) ($1.00 \times 10^{-3}\text{ Pa}\cdot\text{s}$). At 4°C, viscosity increases to 1.56 cP—a 56% increase in fluid shear resistance.
2. Stokes' Settling Velocity: According to Stokes' Law, particle settling velocity ($v_s$) is inversely proportional to fluid viscosity:

$v_s = \frac{g(\rho_p - \rho_w)d^2}{18\mu}$

Where $g$ is gravitational acceleration, $\rho_p$ is particle density, $\rho_w$ is water density, $d$ is particle diameter, and $\mu$ is dynamic viscosity. The 56% increase in viscosity reduces settling velocity by approximately 35%, meaning floc that settles cleanly in autumn will remain suspended in spring, carrying over onto filters unless chemical dosing or basin hydraulic loading is adjusted.

1. Hydrolysis Retardation: The chemical hydrolysis of metal coagulants (alum and ferric salts) is endothermic. In cold water, trivalent aluminum ions take significantly longer to form insoluble amorphous hydroxide precipitates $[\text{Al(OH)}_3\text{(s)}]$, leading to pinpoint floc formation, coagulant carryover, and elevated residual dissolved aluminum in finished water.

---

3. Seasonal Reservoir Stratification and Turnover

Deep raw water storage reservoirs in Colorado undergo seasonal thermal stratification due to water's unique property of reaching maximum density at 3.98°C (~4°C).

Summer Stratification Structure

During late spring and summer, solar radiation heats the upper water column while deeper water remains cold, establishing three distinct thermal zones:

1. Epilimnion: The warm, less dense upper layer (18–24°C). Characterized by high dissolved oxygen (DO > 7.0 mg/L), intense sunlight penetration, active photosynthesis by algae, and elevated pH (8.0–9.0) due to biological consumption of dissolved carbon dioxide.
2. Metalimnion (Thermocline): The transitional middle layer where water temperature drops rapidly with increasing depth—defined as a temperature decrease of at least 1.0°C per meter of depth ($0.55^\circ\text{F/ft}$). This sharp density gradient acts as a physical barrier preventing vertical mixing between top and bottom layers.
3. Hypolimnion: The cold, dense bottom layer (4–8°C). Sealed off from atmospheric reaeration and sunlight, microbial decomposition of settling organic matter consumes all available oxygen, driving the hypolimnion into anaerobic/anoxic conditions (DO < 1.0 mg/L).

+-------------------------------------------------------------+
| EPILIMNION: Warm (18-24°C), High DO, Sunlight, Algae, pH 8-9 |
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
| THERMOCLINE / METALIMNION: Rapid Temp Drop (>1°C per meter) |
+=============================================================+
| HYPOLIMNION: Cold (4°C), Anoxic (DO ~ 0 mg/L), High Fe2+,    |
|              Mn2+, H2S, NH3, Low pH (6.0-6.8)               |
+-------------------------------------------------------------+

Chemical Consequences of Anoxia in the Hypolimnion

Under reducing conditions (negative oxidation-reduction potential / ORP), insoluble sediment minerals dissolve into the water column:

• Insoluble Ferric Iron ($\text{Fe}^{3+}$) reduces to soluble Ferrous Iron ($\text{Fe}^{2+}$) ($>0.3\text{ mg/L}$ causes reddish-brown staining and turbidity).
• Insoluble Manganic Manganese ($\text{Mn}^{4+}$) reduces to soluble Manganous Manganese ($\text{Mn}^{2+}$) ($>0.05\text{ mg/L}$ causes black staining and distribution system complaints).
• Sulfate ($\text{SO}_4^{2-}$) reduces to Hydrogen Sulfide ($\text{H}_2\text{S}$), generating rotten-egg odors, extreme chlorine demand, and corrosive acids.
• Nutrient Release: Soluble orthophosphate and ammonia ($\text{NH}_3$) release from benthic sediments into the bottom water.

Fall and Spring Turnover (Overturn)

In late autumn, atmospheric cooling lowers epilimnion temperature. As surface water approaches 4°C, it reaches maximum density and sinks. Combined with high seasonal mountain winds, the density gradient across the thermocline collapses, triggering destratification (fall turnover). The entire lake volume circulates, suddenly distributing anoxic, mineral-laden, sulfide-rich bottom water across all intake elevations. Operators must immediately increase oxidant dosing (potassium permanganate, chlorine dioxide, or ozone) and adjust coagulant feed to prevent process failure.

---

4. Algal Blooms and Taste and Odor Compounds

In nutrient-enriched raw water reservoirs, warm summer temperatures and sunlight trigger blooms of cyanobacteria (blue-green algae), including Anabaena, Microcystis, Aphanizomenon, and Oscillatoria.

Key Secondary Metabolites

Cyanobacteria produce two notorious organic metabolites responsible for widespread consumer complaints:

1. 2-Methylisoborneol (MIB): Imparts an intense musty, chemical odor.
2. Geosmin ($C_{12}H_{22}O$): Imparts a distinct earthy, damp-soil odor.

Both compounds possess an astonishingly low Human Odor Threshold Concentration (OTC) of 5 to 10 nanograms per liter (ng/L or parts per trillion).

$\text{1 ng/L} = 0.000001\text{ mg/L} = 10^{-6}\text{ ppm}$

Conventional coagulation, sedimentation, and granular media filtration achieve $<20%$ removal of dissolved MIB and geosmin because these tertiary alcohols are non-polar, soluble, and resistant to standard oxidants (chlorine and chlorine dioxide). Effective removal requires Powdered Activated Carbon (PAC) adsorption, granular activated carbon (GAC) contactors, or advanced oxidation processes (Ozone + Hydrogen Peroxide / $\text{O}_3/\text{H}_2\text{O}_2$).

---

5. Raw Water Intake Structures and Preliminary Treatment

Raw water enters the water treatment plant through specialized intake infrastructure engineered to protect downstream pumps and optimize initial water quality.

Multi-Level Intake Towers

Reservoir intake structures feature multiple ports at varying elevations. This configuration empowers the operator to select the highest-quality water stratum:

• Withdraw from upper-middle levels during summer to avoid surface algae/scum/debris while staying well above the anoxic hypolimnion.
• Withdraw from intermediate depths during winter to avoid floating surface frazil ice and freezing surface temperatures.

Bar Screens and Presedimentation Basins

1. Trash Racks and Bar Screens: Positioned at the intake portal. Coarse trash racks (openings 2–4 inches) stop logs, branches, and large boulders. Fine bar screens (openings 0.5–1.0 inch) remove leaves, aquatic vegetation, and fish. Cleaning is performed with manual rakes or automated mechanical traveling screens.
2. Presedimentation Basins (Sand/Grit Traps): Located ahead of raw water lift pumps. With detention times of 15 to 60 minutes and flow velocities reduced to $<0.5–1.0\text{ ft/s}$, dense mineral grit, gravel, and heavy coarse silts settle out by simple gravity, preventing severe abrasive wear on pump impellers, valves, and flow meters.
3. Frazil Ice Mitigation: In supercooled mountain streams ($-0.05^\circ\text{C}$ to $0.00^\circ\text{C}$ under turbulent open flow), frazil ice forms tiny, highly adhesive needle crystals that coat and completely blind intake trash grates within minutes. Plants combat frazil ice by maintaining intake port velocities below $0.5\text{ ft/s}$, utilizing compressed air bubbling systems, or installing low-voltage electric resistance heating elements on intake bar racks.