Disposal of Septic Tank Effluent in Calcareous Sands

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Abstract
Perth, Western Australia, has 430 000 households with septic tanks discharging through sandy soils into the groundwater, which is pumped for domestic irrigation and public water supply. This study investigated the suitability of calcareous sands (Xeropsamment), one of Perth's major soil types, for treatment of septic tank effluent. Calcareous sands adjacent and below two septic tank systems were sampled to a depth of 8 m and the soil pH, soil N, and soil P contents were measured. The soil solution was sampled by an immiscible displacement technique and analyzed for N and P. Most of the NH4‐N in the effluent was oxidized to NO3‐N in the unsaturated zone just below the slime layer. Nitrate (NO−3) was present in the soil solution up to a concentration of 50 mg L−1 down to the maximum depth sampled of 8 m. The acidity produced during nitrification of the NH4 dissolved carbonates from the soil changing the soil pH. Below one system, the carbonates were completely removed from the first 4 m of soil lowering soil pH from 9.5 to 5.2. The NO3 in the soil solution was used to monitor the effluent flow pattern, which below one system had significant lateral flow indicating development of preferred flow paths. The soil P was increased by sorption and precipitation of P from the soil solution until it reached an equilibrium. At equilibrium the soil solution P was at the same concentration as in the effluent and the P sorbed on the soil was at a maximum for that concentration. Further down the profile where the sorbed soil P was not at its maximum, all the P was sorbed from the soil solution in a vertical distance of 0.5 m. The suitability of calcareous sands and limestone areas for acidic waste disposal may be limited by rapid dissolution of carbonate and the consequent release of previously sorbed P. Monitoring of effluent disposal in these calcareous soils is made more difficult by the development of preferred flow paths.

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