Biological activity occurring in subsurface regions below the soil zone may be of considerable importance in determining the fate and effect of pollutants in ground water, but this possibility has received little previous attention. This paper comprises a discussion of subsurface biological activity in regard to ground-water pollution as reflected by available literature references. The subsurface environment is discussed in terms of factors likely to be of greatest significance in regard to the development of biological systems, and previous investigations of subsurface microbial activity are reviewed. Available information indicates the presence in the upper continental crust of the earth of numerous regions, particularly those of sedimentary origin, which are probably suitable habitats for many microbial species. Previous investigations of subsurface microbial activity clearly show the presence of diverse microbial populations in many subsurface regions below the soil zone. Hence, microbial activity appears both possible and probable in most subsurface regions of importance in regard to ground water. Further elucidation of the extent and nature of microbial activity in subsurface regions is needed in developing methods for predicting the impact on ground-water quality of pollutants released into the earth's crust.

Major findings are presented from a one-year operational investigation on the use of algae to remove nitrogen from subsurface agricultural tile drainage in the San Joaquin Valley of California. The objectives of the study were to: (1) refine the design criteria, determined in a preliminary investigation, (2) develop operational procedures, and (3) recommend a design for a prototype algal nitrogen removal process.

The report is one of a series which presents the findings of intensive interagency investigations of practical means to control the nitrate concentration in subsurface agricultural waste water prior to its discharge into other water on such areas as the San Joaquin Valley. As a result of the application of large quantities of water to relatively slowly permeable stratified soils, the west side of the San Joaquin Valley now has large areas with groundwater at rootzone depths. Wherever subsurface drains have been installed to control this groundwater, the drainage effluent has had high nitrate concentrations. Large quantities of inorganic nitrogen fertilizers are applied annually and the assumption prevails that fertilizer is the major source of nitrates in the drainage water. The study was designed to evaluate this assumption and to derive, if possible, practical answers regarding the role of on-farm practices in controlling nitrate out-put from the agricultural lands. Examined are the nitrogen budget and methods for reducing the quantity of nitrates in the drainage effluent by modifications in type or use of fertilizers, practicespractives, or drainage techniques.