Earthen Cover Design, Casa Grande, Arizona

Situation

The site, an Arizona Water Quality Assurance Revolving Fund site, is approximately 5 miles northwest of Casa Grande, Arizona. The former chromium disposal pits occupied a 70-foot by 80-foot area located in the center of the 2.5 leased acres. The area impacted by the disposal of chromium solutions into these pits covered approximately 20,600 square feet. Soil with chromium concentrations approaching or exceeding the 1,500 milligrams per kilogram action level established by ADEQ are contained in this area.

Hydro Geo Chem's Approach

The cover was designed for the site to provide the following controls:

1) Contaminant containment: Containment of the soils containing chromium concentrations equaling or exceeding the action levels established by ADEQ by covering the impacted area.

2) Moisture control: Prevent precipitation falling on the cover from ponding and infiltrating into the contamination zone. Provide sufficient cover thickness to allow evapotranspiration to deplete excess moisture.

3) Flood and erosion control: Construct an engineered diversion ditch, grade area, and provide soil-cemented and rock covers to prevent shifting arroyos from inundating the cover and loss of cover material by erosion.

4) Dust control: Vegetation to control dust from the cover and surrounding areas.

5) Capillary rise control: Construct the cover with a capillary barrier to eliminate the upward movement of contaminants to the land surface by capillary rise.

The following tasks were completed to obtain the desired controls:

• The pits were filled in to match the original grade which slopes approximately 1% to the south-southwest.

• The soils and fill were compacted to minimize settling of the cover.

A 1-foot-thick capillary barrier consisting of river-run gravel extended across the entire area of soils equaling or exceeding the ADEQ action level. The gravel material met the following grain size specifications:

• 95 to 100% passing 1-inch sieve
• 0 to 20% passing ½-inch sieve
• 0 to 20% passing -inch sieve

High survivability filter fabric (geotextile) was placed above and below the gravel layer to prevent the layer from being infiltrated with fine grain-size soil material. The capillary barrier controls capillary rise. The barrier and earthen cover is of sufficient areal extent to contain the impacted soils.

A soil cover with a minimum thickness of 32 inches was placed on top of the capillary barrier. Infiltration analyses of the earthen cover borrow material indicated that 32 inches was sufficient to allow evapotranspiration to deplete excess moisture before it could infiltrate into the impacted soil. The final grade of the cover prevents surface water from ponding or inundating the site and directs surface water downgradient of the cover.

The engineered drainage ditch was designed with sufficient capacity, slope, and durability to contain and drain surface waters during large flood events downslope of the cover. A 6-inch-thick cement layer was constructed on the cover edge of the diversion ditch to provide erosion control. This, along with the gradual 4:1 slope of the cover edge, was designed to minimize erosion. The disturbed areas were seeded with hydromulch to control dust and minimize erosion.

Results

A recent survey confirmed the following design components:

• The subgrade is at a similar level and grade as the undisturbed area. No depressions are present across the impacted area and the slope is 1% grade to the south-southwest.

• The impacted area is covered by the capillary barrier and the capillary layer is approximately 1 foot thick.

• The concrete bank protection is constructed at 4:1 slope, the toe is approximately 3 feet below the natural grade, and the bank protection has approximately 2 feet of freeboard.

• The earthen cover is at least 32 inches thick over the capillary barrier and does not have any depressions or low areas.

• The cover slopes towards the edge. The edges slope at a 4:1 grade and the diversion channels have a 1% grade to the south end of the site.

• The monitoring well casings have been extended and resurveyed to allow on-going water level measurements and sampling.