decrease  in NH4-N suggests that nitrification was the major

mechanism of NH4-N removal.

     The high solubility of the nitrate anion allows it to move

freely with groundwater.     If the nitrates enter an anaerobic

environment    in   which    organic   material    is   available,

denitrification, the reduction of nitrate to nitrogen gas, may

occur.  However, significant denitrification is unlikely to occur

in   a  well-aerated  sandy   subsoil  or  in  a  carbon-deficient

groundwater (Walker et al., 1973).

     Nitrate-nitrogen leaching from on-site sewage disposal systems

has been shown to threaten both surface and groundwater quality in

unsewered areas of the United States       (Lamb et. al., 1989).

Nitrates may contribute to the eutrophication of surface waters and

they have also been linked to cases of methemoglobinemia in infants

(U.S. EPA, 1975). Consequently, the U.S. Environmental Protection

Agency and World Health Organization drinking water standard for

nitrate is 10 mg/l as nitrate-nitrogen       (45 mg/l as nitrate)

(Kaplan, 1987).

      Nitrate contamination of groundwater from septic tanks has

been documented.   Walker et. al. (1973) found that soil disposal

systems of septic tank effluents in sands added significant

quantities    of   nitrate  to   the   underlying   ground   water.

Concentrations as high as 40 mg/l of nitrate-nitrogen were found in
the upper 30 cm of aquifer adjacent to the systems.      Relatively

large areas of .2 ha (0.5 acre) down gradient were needed before

concentrations were lower than the standard 10 mg/i.