1.1 Surface Water and Groundwater Systems

Why This Topic Matters for the Exam

Understanding the distinction between surface water and groundwater systems is foundational for any water treatment operator. The source of raw water dictates the type of treatment processes required, chemical dosing, monitoring frequencies, and regulatory compliance. On the certification exam, you will be expected to analyze the differences in water quality parameters between these two sources, understand the mechanics of well hydraulics—specifically well drawdown, cone of depression, and specific capacity—and identify the mechanical components of a groundwater well system.

Surface Water vs. Groundwater Characteristics

Water sources are broadly categorized into surface water and groundwater. Surface water includes rivers, lakes, reservoirs, and streams. It is directly exposed to the atmosphere and surface runoff, which makes it highly dynamic. Turbidity (a measure of water clarity caused by suspended solids) fluctuates rapidly in surface water in response to rain events and wind. Surface water also has higher levels of organic matter, pathogens (such as Giardia and Cryptosporidium), and dissolved oxygen, but lower mineral content than groundwater.

Groundwater, found in underground aquifers, is filtered naturally as it percolates through soil and rock. As a result, groundwater typically features low turbidity, consistent temperature, low organic matter, and minimal pathogen contamination. However, because it spends years in contact with underground geological formations, groundwater has significantly higher mineral content (specifically calcium, magnesium, iron, manganese, and dissolved gases like carbon dioxide and hydrogen sulfide).

Here is a comparison of typical raw water characteristics:

Groundwater Hydraulics and Well Drawdown Concepts

To extract groundwater, operators must understand well hydraulics. When a well is not pumping, the water level in the well matches the surrounding water table. This is called the static water level. Once the pump starts, water is drawn from the surrounding aquifer, and the water level inside the casing drops. The level at which the water stabilizes during active pumping is the pumping water level.

The difference between the static water level and the pumping water level is the drawdown. $\text{Drawdown} = \text{Static Water Level} - \text{Pumping Water Level}$ As water flows toward the well, it creates a depression in the water table that resembles an inverted cone. This is known as the cone of depression. The lateral extent of this cone, where the water table is lowered by pumping, is the zone of influence. If two wells are placed too close together, their zones of influence may overlap, causing well interference, which reduces the yield of both wells and increases pumping costs.

Well Yield and Specific Capacity

Two critical performance metrics for wells are yield and specific capacity. Well yield is the volume of water per unit of time that a well can produce under continuous pumping (typically expressed in gallons per minute, or gpm). Specific capacity is a measure of well efficiency, calculated as the well yield divided by the drawdown: $\text{Specific Capacity} = \frac{\text{Well Yield (gpm)}}{\text{Drawdown (ft)}}$ A decrease in specific capacity over time indicates well degradation, such as screen encrustation, bacterial biofouling, or sand plugging.

Example Calculation: A well has a static water level of 45 feet. After pumping at a rate of 400 gpm for several hours, the pumping water level stabilizes at 70 feet. What is the specific capacity?

1. Calculate Drawdown: $\text{Drawdown} = 70\text{ ft} - 45\text{ ft} = 25\text{ ft}$
2. Calculate Specific Capacity: $\text{Specific Capacity} = \frac{400\text{ gpm}}{25\text{ ft}} = 16\text{ gpm/ft}$

Well Components and Construction

A public water supply well consists of several key structural components:

• Well Casing: The steel or plastic pipe lining the borehole, preventing collapse and keeping out shallow, contaminated water.
• Well Screen: A perforated pipe section placed in the aquifer that allows water in while keeping sand and gravel out.
• Gravel Pack: Clean, graded sand or gravel placed between the screen and borehole wall to filter particles and increase permeability.
• Sanitary Seal: A seal at the top of the casing preventing contaminants from entering the well.
• Pitless Adapter: A device allowing the discharge line to exit the casing below the frost line to prevent freezing while maintaining a seal.
• Grout: A cement slurry pumped into the annular space to prevent surface water migration down the casing.

Realistic Exam Scenarios

Scenario A: An operator notices a gradual decline in the specific capacity of a groundwater well over a six-month period, while the static water level remains stable. This indicates that the aquifer is not depleting, but rather the well itself is failing. The operator should schedule a well rehabilitation (e.g., acid treatment for carbonate scale or physical surging to clear the screen). Scenario B: Following a heavy rain event, a surface-water-influenced well (known as groundwater under the direct influence of surface water, or GWUDI) shows a rapid rise in turbidity and a positive test for coliform bacteria. Because the well exhibits surface water characteristics, regulations dictate that it must be treated with filtration and disinfection, just like a surface water source.