Crop fields generally rely on either existing sources of surface water or high capacity wells and pumps to stay irrigated during the driest parts of the year. Removing this water from the natural system of surface to underground water table supply can result in a slow loss of volume over time. Encouraging water to soak into the ground in the fields and through designated recharge points is all part of a properly designed drainage system. Water runs through properly designed drainage systems too quickly to absorb into the soil through the walls of ditches and canals. By adding dry wells, soil distribution furrows, sprayers, and even buried pipes, you can recharge your local water table without any risk of contamination.
The majority of water table recharging plans involve some kind of subsurface discharge. Instead of holding the water at the surface in an unlined canal or pond and waiting for it to slowly seep through the soil, buried perforated drain tile pipes, dry wells, or other underground features are used to provide subsurface drainage. This reduces evaporation and ensures as much water as possible reaches the water table for recharge. If the water isn’t perfectly clean by the time it’s discharged below the surface, the subsurface features should be installed shallowly. This allows the water to trickle through as much soil and rock before it reaches the water table, providing a final filtration effect for free.
Water Table Basics
The water table is simply the depth at which water completely saturates the soil and stone below the surface. Some parts of the country have very high water tables, meaning that digging a hole just a few feet into the ground will cause it to fill up rapidly with water. Other areas have water tables so deep they’re hard to reach with conventional well drilling equipment. Regardless of its natural depth, the water table rises and falls over time as rainfall recharges it and pumping or use of surface water depletes it. If you continue to pump a water table without ensuring there’s enough of a recharge rate from natural rainfall and returned runoff, you can deplete an area’s supply and leave it basically impossible to irrigate without expensive trucking services to haul in water. Recharging ground water supplies with careful discharging techniques is essential to keep the water table high and accessible for the needs of your business and everyone else in the area.
Processing and Treatment Options
Releasing waste water directly into the water supply, even underground where soil and sand offer a natural filtering effect, can lead to unexpected issues. Contaminants that aren’t filtered out by the ground can become trapped in the aquifer and travel far farther than expected. If the water table is shared with local homes, waste water discharge can result in unsafe drinking water. Recharging the water table is admirable and even required in many parts of the country, but it must be done with care to avoid further damage. Your options for processing and treating waste water before releasing it for underground absorption include:
- Settling to accumulated particles and dissolved solids at the bottom of a tank or pond
- Flocculation, which uses additives to encourage particles to “flock” together and clump into heavier chunks that settle faster
- Multiple filtration media methods, including activated carbon, sand, clay particles, and microorganisms
- Reverse osmosis treatment, in which a membrane is used to separate the finest particles and contaminants from the water
- UV and ozone applications, two powerful options for killing bacteria and viruses
- Aging, which relies on time to allow nature to slowly reclaim and settle out unwanted ingredients.
If you can’t process your waste water to a point where it would be safe for subsurface discharge, you may need to hold it in a lined well instead. This keeps the contaminated water underground and away from any potential contact with animals or people, but it doesn’t let the water go anywhere either. Lined wells don’t recharge the water table, but they can protect it from contamination when there’s concentrated waste that is too costly or difficult to process. In general, the EPA states that no discharge well or trench owner is allowed to release any waste products that would affect the water table in a way that’s prohibited by the National Primary Drinking Water Quality Standard.
Leaching Pits and Trenches
Trenches and pits designed to release water slowly through seepage are the simplest forms of ground water discharge available today. They’re generally dug underground and kept open with vault-like plastic or concrete forms that hold the soil above out of the trench. Some are built on the surface instead and should be covered with an impermeable layer on top to prevent evaporation and volume gain through rainfall. Naturally, the bottoms and sides of these trenches and pits are left unlined since seepage is necessary to achieve a slow rate of discharge. Any trenches and canals feeding waste water to these discharge pits should be lined to ensure water doesn’t start seeping into the ground until it reaches the right point for transition. When built on the surface, these pits can superficially resemble other types of holding and processing ponds. However, the pits and trenches are almost always designed to be much deeper than they are wide or long at the surface. This maximizes subsurface release while minimizing surface-level loss through evaporation and splashing.
Dry Wells, Septic Tanks, and Other Options
Dry wells are slightly more complex leaching pits, while septic tank designs similar to those used for residential sewage can be put to use for practically type of waste water discharge. Other structures commonly used for subsurface water table recharge include:
- Underground or surface cesspools
- Drainfields, used either alone or in conjunction with septic tanks
- Direct drains and collection basins that connect to pits or dry wells
- Advanced periodic release wells with high-tech monitoring equipment.
All of these subsurface discharge options require routine maintenance, including unclogging and solids removal in many cases. Use durable liners around the banks and edges of these features to ensure there’s no erosion from foot traffic during maintenance chores. Even if the pit or trench itself needs to remain unlined for maximum seepage, there’s still plenty of ways to use liners and covers from BTL Liners around and over the features.