1.1 Hydrologic Cycle, Watersheds & Water Sources

The Hydrologic Cycle and Water Movement

Water is constantly recycled through the hydrologic cycle, driven by solar energy and gravity. Water molecules cycle through three distinct operational phases: atmospheric, surface, and subterranean. Understanding these continuous pathways enables operators to anticipate seasonal raw water fluctuations, contaminant transport mechanisms, and catchment vulnerabilities.

                    [ Precipitation ]
                      /          \
                     /            \
        [ Overland Runoff ]     [ Infiltration ]
               |                       |
        (Surface Waters)        [ Percolation ]
               |                       |
        [ Evaporation ]         (Groundwater Aquifers)
               \                      /
                \--- [ Transpiration ]

Core Hydrologic Mechanisms

• Evaporation: Solar thermal radiation converts liquid water from open lakes, reservoirs, rivers, and oceans into atmospheric water vapor. Evaporation leaves behind non-volatile minerals, concentrating dissolved salts in arid surface basins.
• Transpiration: Terrestrial vegetation extracts soil moisture through root systems and discharges water vapor into the atmosphere via foliar stomata. Together, evaporation and transpiration are quantified as evapotranspiration.
• Condensation: Ascending water vapor cools in the upper troposphere, nucleating onto microscopic aerosol particles to form liquid droplets or ice crystals that consolidate into clouds.
• Precipitation: Atmospheric moisture condenses beyond cloud suspension thresholds and falls to Earth as rain, snow, sleet, or hail. Precipitation is the direct input volume for surface watersheds and subterranean aquifer recharge.
• Infiltration: The initial process of precipitation wetting the land surface and passing vertically across the soil-air interface into upper soil strata.
• Percolation: The continued downward gravitational migration of infiltrated water through unsaturated soil horizons (the vadose zone) until it reaches the saturated zone, replenishing groundwater aquifers.
• Overland Runoff: Precipitation exceeding soil infiltration capacity flows over the ground surface into natural collection channels, streams, rivers, and impoundments. Runoff mobilizes topsoil silt, clay, synthetic fertilizers, pesticides, industrial residues, and pathogenic microorganisms, creating sudden turbidity and microbial spikes at surface water intakes.

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Surface Water vs. Groundwater Sources: Comparative Operational Profiles

Public water systems rely on surface water, groundwater, or conjunctive-use blends. Each supply type presents distinct chemical, physical, and microbiological profiles that govern plant design and daily operating procedures.

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Aquifer Hydrogeology and Well Hydraulics

Groundwater exists in subterranean geological formations capable of storing and transmitting usable quantities of water, termed aquifers. Operators must understand the physical structure and hydraulic response of these formations.

Unconfined vs. Confined Aquifers

1. Unconfined (Water Table) Aquifers: Formations where groundwater is in direct hydraulic contact with the atmosphere through permeable soil pores. The upper boundary is the water table (phreatic surface), which rises and falls based on local precipitation recharge. These aquifers are highly vulnerable to shallow surface contamination (septic leachates, agricultural chemicals, surface spills).
2. Confined (Artesian) Aquifers: Water-bearing formations trapped between upper and lower confining geological strata of low permeability, termed aquitards or aquicludes (e.g., dense clay or un-fractured shale). The water is under hydrostatic pressure. When a well penetrates a confined aquifer, water rises above the bottom of the confining layer to a level termed the piezometric (potentiometric) surface. If the potentiometric surface exceeds ground elevation, the well is a flowing artesian well.

Well Diagnostic Calculations

Operators monitor static and dynamic water levels to diagnose well efficiency, encrustation, and aquifer depletion.

Ground Surface  ===========================================
                |                                         |
                |                                         v
                |                          Static Water Level (SWL)
                |                                         ^
                | <--- Drawdown (DD) --->                 |
                |                                         v
                |                          Pumping Water Level (PWL)
                |                     
    ============|============ (Pump Intake) ===============
                |                     
                v Bottom of Well

• Static Water Level (SWL): The vertical distance (in feet) from the ground surface (or reference measuring point) to the water level in the well casing when the pump is shut off and the aquifer has fully recovered.
• Pumping Water Level (PWL): The stabilized vertical distance (in feet) from the measuring point to the water level during continuous pumping at a given discharge rate.
• Drawdown ($DD$): The net distance the water level drops during pumping: $DD = PWL - SWL$
• Specific Capacity ($SC$): The well yield generated per unit of drawdown, expressed in gallons per minute per foot of drawdown ($gpm/ft$): $SC = \frac{\text{Well Discharge Rate }(Q\text{ in }gpm)}{\text{Drawdown }(DD\text{ in }ft)}$
• Cone of Depression: A three-dimensional, inverted conical depression in the water table or potentiometric surface centered around an active pumping well.
• Zone (Radius) of Influence (ZOI): The radial horizontal distance from the well casing to the outer edge of the cone of depression where drawdown diminishes to zero.

Class II Operational Diagnostic: A gradual decline in specific capacity ($gpm/ft$) at a constant pumping rate indicates well screen encrustation (iron/manganese bacteria biofouling or calcium carbonate scaling), sand pack binding, or regional water table depletion. If drawdown increases while pumping rate drops, well redevelopment (chemical acidizing, surging, or chlorination) is indicated.

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Groundwater Under the Direct Influence of Surface Water (GWUDI)

The Safe Drinking Water Act (SDWA) and the Surface Water Treatment Rule (40 CFR § 141.70) define Groundwater Under the Direct Influence of Surface Water (GWUDI) as:

Any water beneath the surface of the ground with significant occurrence of insects or other macroorganisms, algae, or large-diameter pathogens such as Giardia lamblia or Cryptosporidium, or significant and relatively rapid shifts in water characteristics such as turbidity, temperature, conductivity, or pH which closely correlate to climatological or surface-water conditions.

Determination Criteria and Testing

1. Hydrogeologic Sensitivity: Wells located within 200 feet of a surface water body, shallow wells with casing depths under 50 feet, or wells drilled in karst limestone, unconfined gravel, or fractured bedrock are automatically flagged for GWUDI assessment.
2. Water Quality Correlation: Continuous monitoring displaying rapid turbidity spikes (e.g., jumping from 0.3 NTU to 5.0 NTU within hours of a storm) or temperature variations exceeding 1°C–2°C that mirror river conditions indicates direct surface hydraulic connection.
3. Microscopic Particulate Analysis (MPA): The EPA consensus method requires filtering large volumes (500 to 1,000 gallons) of raw well water across a 1-micrometer wound polypropylene cartridge filter over a 24-hour period following a heavy runoff event. The caught residue is microscopically analyzed for surface water bio-indicators:
   • Giardia lamblia cysts and Cryptosporidium oocysts
   • Chlorophyll-bearing algae, diatoms, and cyanobacteria
   • Aquatic rotifers, nematodes, and crustaceans
   • Terrestrial plant debris and insect parts

Regulatory Consequences

If a well is formally classified as GWUDI, it loses its status as a simple groundwater system. The utility is legally mandated to install full surface water filtration (conventional, direct, or approved membrane filtration) and provide disinfection that meets the SWTR performance criteria: 3-log (99.9%) removal/inactivation of Giardia lamblia and 4-log (99.99%) removal/inactivation of enteric viruses.