OVERBURDEN SAMPLING AND ANALYTICAL TECHNIQUES

Other Overbuden Testing Methods

Supplementary tests in addition to acidity/alkalinity analyses may be required or recommended by regulatory authorities, particularly in the Western United States. Additional analyses are usually conducted for selected trace elements or general indicator parameters that have been determined by the RA to be of statewide or area-wide importance. These may include:

1. Salinity and electrical conductivity.
   
2. Sodium and sodium-adsorption ratio (SAR).
   
3. Selenium.
   
4. Boron.
   
5. Plant nutrients, such as phosphorus and nitrogen.
   

Salinity -

Salinity is a measure of the soluble salts present in the overburden. In addition to sodium, it includes chlorides and sulfates of potassium, calcium and magnesium. These salts are easily leachable from exposed soil or overburden material. Excess salinity can increase osmotic pressure in the root zone, resulting in less water uptake by the plants. Salinity can also impact the suitability of water for irrigation and livestock watering. Salinity hazard is assessed by measuring the electrical conductivity of an aqueous extract of the overburden. Many western states have suitability criteria for electrical conductivity/salinity.

Sodium -

Sodium (Na) is a common constituent in unleached overburden. It can be present in the form of salts in solution, or adsorbed onto clays. Sodium is highly mobile and can readily leach from exposed spoil. Once mobilized it can migrate upward from the spoil into the root zone. Sodium can also cause dispersion and swelling of clays, especially montmorillonite. High levels of sodium in the root zone causes deterioration of soil structure that restricts water movement, aeration and plant development. The suitability of water supplies for irrigation and livestock watering can also be affected by high levels of sodium. The common methods used for assessing sodium hazard in the overburden are SAR and Exchangeable Sodium Percentage (ESP). The SAR is a ratio used to express the relative activity of sodium ions in exchange reactions with soil where ionic concentrations are expressed in milliequivalents per liter.

Selenium -

Selenium (Se) is a trace element that is commonly associated with pyrite and has similar geochemistry to sulfur. Although selenium is an essential nutrient, it can also be extremely toxic. Selenosis (selenium toxicity) is a hazard to livestock when excess selenium accumulates in either water or plants. Selenium concentrates in organic material and in coal; selenium can be enriched nearly one hundred times its average crustal abundance (Herring, 1991). Care must be taken not to place selenium overburden material in the recharge zone where it could contaminate underlying aquifers. In the western United States, the best zone for placing selenium overburden is in the subsurface non-saturated zone (Brown, 1991).

Boron -

Boron (B) is a widely distributed trace element in many rock types and has a tendency to concentrate in arid soils. The range for boron toxicity and deficiency in soils is narrow, only a few tenths milligram per liter (parts per million) (Bohn and others, 1979). Boron concentrations of irrigation waters are particularly important because many crops are susceptible to even extremely low concentrations of this element.

The ESP is the measure of cation exchange capacity (CEC) occupied by sodium. Most western states have developed their own criteria and guidelines for SAR and ESP limits.