The first step in deciding if the surfactant technique is appropriate for a specific site is determining if it is potentially cost effective. Assume for the moment an approximate annual material cost for surfactant of $600 per acre. Add to that the cost of three applications, the expense of which will vary from site to site. One should consider, for example, whether the surfactant will be applied with equipment already on-site and using available personnel time or with contracted equipment and manpower. For potential savings, assume a 60-pct decrease in acidity; this will reduce neutralization costs by about the same amount. If a 90-pct decrease in sludge accumulation is further assumed, one can estimate an associated annual savings based on the frequency with which the pond is normally cleaned. Other treatment costs, such as equipment and operator expenses, will generally not decrease significantly since the volume of water to be treated stays the same.

Since the surfactant must reach the pyritic material to be effective, the presence of intervening overburden or topsoil will clearly reduce the effectiveness of the treatment. If the cover is highly permeable, treating with somewhat higher levels of surfactant will compensate; if not, the surfactant will probably all be adsorbed by the nonpyritic material or will be washed away in surface runoff. Several mine operators have recently tried to solve this problem by applying the anionic surfactant solution through shallow wells; it is too early to judge if they have been successful.

If quick results are required, two other factors must be considered: hydrologic flow-through time and ground water pools. Flow-through time is the time it takes for rainwater to infiltrate through the mine material and emerge in a spring

or seep. In mine spoil, flow-through times of over a year have been measured (19). In such a case, improvement in water quality at the discharge point cannot occur faster than water flows through the material.

Ponding of acid water on the old mine floor or in a refuse area also retards or

tion. The

masks the effect of reduced acid producpresence of such a pool can sometimes be surmised by a steady acid load in the base flow during dry periods and only slight changes in the acid load during higher flows. If the pool volume is significant, it could be years before

a decrease in acid production is reflected by more than a gradual improvement in water quality. If faster results are desired, alkalinity must be added to the pool in sufficient quantities to neutralize all of the acidity present.

APPLICATION PROCEDURES

If, based on the above considerations, the site appears appropriate for the surfactant treatment, the obvious question is how much surfactant is required. As described earlier, the concentrations needed to kill T. ferrooxidans in small volumes of coal refuse are quite low (25 to 40 mg/L). Such an application would

be ineffective at an actual mine site because it would fail to reach most of the oxidizing pyrite. It is necessary to determine the adsorption capacity of the material being treated and to compen

sate for that adsorption with extra surfactant.

A simple laboratory procedure has been developed to provide an estimate of adsorption potential for overburden and coal refuse. A representative sample of the material to be treated is placed in a large, tared Buchner funnel, tamped to a uniform depth of 2 in, and weighed. A surfactant, such as SLS, is applied evenly over the material at approximate loads of 60 mg SLS/kg of coal refuse. Typical SLS concentrations range from 300 to 30,000 mg/L and are selected on the basis of the anticipated infiltration rate.