2.2 Groundwater Sources, Wells & GWUDI Determinations

2.2 Groundwater Sources, Wells & GWUDI Determinations

Source Water Characteristics is a full content area on the standardized water treatment exam, and in South Carolina the majority of public water systems are groundwater systems. A Group I or Group II plant may be nothing more than a well, a hypochlorite feed, and a hydropneumatic tank — but the operator is still expected to understand what the aquifer is doing and what the well is telling them.

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1. Aquifer Types

In a confined aquifer the water rises in the casing above the top of the aquifer to the potentiometric surface. If that surface is above ground level, the well flows without pumping — a true artesian well.

Typical groundwater quality signature: low turbidity, low or absent bacteriological contamination, stable temperature, but frequently elevated hardness, iron, manganese, carbon dioxide, and hydrogen sulfide, and sometimes naturally occurring radionuclides or arsenic. Groundwater is often anoxic, which is precisely why iron and manganese stay dissolved until you aerate or oxidize.

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2. Well Construction and Protection

A properly constructed well is the first barrier against contamination. Key elements:

• Casing — seals the borehole from surface water and shallow contaminated zones.
• Annular grout seal — cement or bentonite filling the space between the casing and the borehole wall. A failed grout seal is a direct conduit for surface contamination and is a common cause of unexplained coliform positives.
• Sanitary well seal / cap — vermin-proof, with a screened, downturned vent.
• Casing extension above grade with the surrounding ground sloped away so surface water drains off.
• Well screen and gravel pack — sized to the aquifer material to admit water while excluding formation sand.

In South Carolina, only licensed well drillers may open or close water wells, with narrow statutory exceptions for government employees performing trained duties, property owners working on their own property, exploration borings, and licensed contractors drilling boreholes for water-level measurement, blasting, or short-term construction dewatering. Well drillers must furnish proof of a $25,000 surety bond to the Board.

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3. The Two Measurements That Matter

Static and pumping levels

• Static water level — the level in the well after it has rested long enough to recover, measured from a fixed reference point.
• Pumping (dynamic) water level — the level while the pump is running at a stated rate.

$\text{Drawdown (ft)} = \text{Pumping Water Level} - \text{Static Water Level}$

Specific capacity — the well's report card

$\text{Specific Capacity} = \frac{\text{Well Yield (gpm)}}{\text{Drawdown (ft)}} \quad \left[\frac{\text{gpm}}{\text{ft}}\right]$

Worked example. A well has a static level of 40 ft. Pumping at 350 gpm, the level stabilizes at 110 ft.

• Drawdown = 110 − 40 = 70 ft
• Specific capacity = 350 ÷ 70 = 5.0 gpm/ft

A year later the same well pumps 350 gpm at a pumping level of 180 ft:

• Drawdown = 180 − 40 = 140 ft
• Specific capacity = 350 ÷ 140 = 2.5 gpm/ft — a 50% loss

That is the diagnostic signal. Specific capacity trending down at a stable static level points at the well, not the aquifer.

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4. GWUDI — When Groundwater Stops Being Groundwater

Groundwater Under the Direct Influence of surface water (GWUDI) is a formal determination by the primacy agency that a well, spring, or infiltration gallery receives water from surface sources quickly enough that surface pathogens can reach it. Indicators include:

• Significant occurrence of insects or other macroorganisms, algae, or large-diameter pathogens such as Giardia lamblia or Cryptosporidium.
• Significant and relatively rapid shifts in water characteristics — turbidity, temperature, conductivity, or pH — that closely correlate with rainfall or nearby surface water conditions.
• Shallow construction, proximity to a stream or lake, or fractured or karst geology providing a short travel path.

Why the determination is a big deal

A GWUDI source is regulated as surface water. That means the Surface Water Treatment Rule applies: the system must provide filtration and disinfection, meet combined filter effluent turbidity limits, and demonstrate CT-based log inactivation credit for Giardia and viruses. Operationally it also moves the plant up the South Carolina group ladder — a well that was a Group II plant requiring a Class D operator becomes a surface water plant in Group IV, V, or VI, requiring Class C, B, or A depending on the treatment technology installed.

Exam framing: if a question describes a well whose turbidity spikes within hours of a rainstorm, or in which Giardia cysts or diatoms are detected, the expected answer involves a GWUDI determination and the imposition of surface water treatment requirements — not simply increasing the chlorine dose.