Erosion and Sediment Control
This article describes best management practices that should be used in developing and implementing erosion and sediment (E&S) control plans for water well drilling and aquifer testing sites. Because site conditions vary greatly, it is recommended that all well drilling contractors meet with the appropriate conservation entities prior to beginning the work to discuss site constraints and to develop the most appropriate E&S plan. Consideration should be given to the terrain, vegetative cover, soil types, underlying geology and proximity to waterways and wetlands.
Residential WellsThe installation of most residential wells typically involves the generation of up to 25 gallons per minute (gpm), including both the drilling operation and the pumping operation. The recommended E&S control measures to address the low flows leaving the well casing involve limited detention and then discharge from the well area overland through vegetated terrain.
Detention time should be provided to cause the well cuttings and solids to drop out of the flow. For wells being drilled at construction sites, this can be accomplished by excavating a shallow trench, 10-15 feet long, immediately downgrade of the well. The downstream edge of the trench should be level to allow water to spill out uniformly over the entire length of the trench. A semi-circle of silt fence or straw bales should be installed downgrade of the trench for further sediment removal.
From the trench, overland flow should be directed as sheet flow across a thickly vegetated area. A minimum of 50 feet of vegetated terrain must be available between the trench and the nearest down slope water conveyance. If adequate vegetation is not available downgrade of the well site, or if the minimum distance to water conveyance is less than 50 feet, E&S measures for medium and high capacity wells described below should be employed. These more rigorous E&S measures for medium and high capacity wells also should be used for all residential or small-sized wells drilled within 200 feet of special protection waterways.
For wells being installed in established, landscaped areas, minimal site disturbance is desirable. An alternative to the trench described above is to use a device on the well casing to divert drillings to a tank truck or a container, which can be hauled from the site for disposal.
Higher-capacity WellsMore rigorous E&S measures are recommended for medium- and high-capacity wells with anticipated flows exceeding 25 gpm. Most larger private wells and public water supply wells will exceed these low flows.
The recommended E&S measures to address the medium to high flows leaving the well casing involve detention in an excavated sump and pumping the settled water through a filter bag.
Adequate detention time can be provided by directing well casing overflow to an excavated sump sized so that the volume of the sump in cubic feet is equal to the flow in gallons per minute. Settled water from the sump is pumped to an appropriately sized filter bag. According to manufacturer's recommendations, the filter bag should be placed on a stabilized area of dense vegetation. If the vegetative cover is not available, the filter bag should be placed on a bed of gravel.
Measures for Aquifer TestingFlows exceeding 500 gpm are possible during aquifer testing of some water supply wells. The clarity of the test flows may vary from very muddy in a limestone aquifer where clay is present, to quite clear in a sandstone aquifer. Measures must be taken to prevent sediment pollution from aquifer tests with turbid water. Precautions also are needed to allow high flows of clear water to discharge from the site without causing accelerated erosion of the landscape.
Muddy water yielded by aquifer testing should be allowed to settle and then pumped through a filter bag. Considering that these flows can be very high, the contractor must utilize large enough filter bags or construct a manifold system using several smaller bags at one time. Care also should be taken to replace the bags promptly when full or when they fail due to a tear in the material.
In a situation where the actual well yields exceed anticipated flows by a considerable amount and the erosion and sediment control measures are inadequately sized, and waterways are threatened with sediment pollution, the operation must be shut down until more appropriate E&S controls are provided.
Clear water flows generated during well yield pumping should be discharged to a watercourse by way of a diversion channel or conduit. The project's E&S plan should provide details on proposed means of transporting the clear water. The channel lining must be designed appropriately for the anticipated velocity. In a similar manner, if a conduit is proposed, supporting hydraulic design information should be provided. Adequate protection against erosion should be placed at the downstream end of the clear water diversion channel or conduit.
It is important to remember that when considering the most appropriate means of controlling erosion and sediment at well sites, recirculating the discharge water into the aquifer will invalidate the aquifer test. Therefore, care must be taken in designing and locating the E&S control measures so that the aquifer test does not induce artificial recharge from discharged well water.