B Potential hazard of liner failure for waste impoundments

Waste impoundments present a risk of contaminating underlying ground water aquifers and surface water that may be fed by these aquifers because of the nutrients and micro-organisms contained in the wastewater. To minimize this risk, NRCS practice standards require that waste impoundments be located in soils of acceptable permeability or be lined. Despite this, risk remains because of the possibility of poor performance of these measures in preventing the movement of contaminants to the ground water. Any of a number of causes could lead to nonperformance of liners. These causes would include such things as not being homogenous with lenses of more permeable material, being constructed with inadequate compaction, having desiccation cracks develop following impoundment emptying, and being damaged during agitation. Flexible membrane liners may fail by such things as cracks, tears, seam separation, or loosened connections. Concrete liners may leak if they crack or joint seals fail. The acceptability of the risk depends on the importance of the underlying aquifer, the location and type of aquifer, and geologic site conditions that may be unforgiving to poor performance.

The seepage protection planned for a waste impoundment should correspond to the risk involved. A thorough geologic investigation is essential as a prerequisite to planning seepage control for a waste impoundment. Special consideration should be given to seepage control in any one of the following conditions:

• Any underlying aquifer is at a shallow depth and not confined.

• The aquifer is a domestic water supply or ecologically vital water supply.

• The site is located in an area of carbonate rock (limestone or dolomite).

Should any of these conditions exist, consideration should be given to the following:

• A clay liner designed and installed in accordance with procedures of appendix 10D with a thickness and coefficient of permeability so that specific discharge is less than 1 x 10-6 centimeters per second.

• A flexible membrane liner over a clay liner.

• A geosynthetic clay liner flexible membrane liner.

• A concrete liner designed in accordance with the criteria for watertight slabs on grade.

The subsurface investigation for a waste impoundment site must be conducted so as to locate any subsurface drainage lines. If found, the lines must either be removed, rerouted, or replaced with nonperforated pipe with watertight joints

Some waste impoundments require foundation drains to lower the seasonal water table to an acceptable depth. These drains must be designed and installed to have an appropriate separation distance from the impoundment liner and outlet in nonsensitive areas. Functional failure of these drains may impact impoundment liner performance. As such, outlets should be guarded from damage and located so they can be inspected for proper operation. Dual outlets should be considered so a backup outlet is available if one fails.

Pumping and agitation, if used, can be destructive to liners, especially soil blanket liners. Plan for pumping and agitation at locations that will not result in damage to liners or for measures that will eliminate the possibility of damage.

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