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Modeling the Impact of Clustered Septic Tank Systems on Groundwater Quality

Liping Pang^a,*, Chris Nokes^a, Jirka imnek^b, Heather Kikkert^a and Ross Hector^a

^a Inst. of Environmental Science & Research Ltd., P.O. Box 29181, Christchurch, New Zealand
^b Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521

View larger version (25K): [in a new window]   Fig. 1. Schematic of the transport domain and conceptual model, including defined boundary conditions.

View larger version (21K): [in a new window]   Fig. 2. Hydrus-2D simulated NO3– plumes developed under the impact of clustered septic tank systems (results at 1000 d).

View larger version (14K): [in a new window]   Fig. 3. Simulated NO3– concentrations in groundwater 1, 4, and 10 m below the water table across the model domain (results at 1000 d).

View larger version (14K): [in a new window]   Fig. 4. Comparison of simulated NO3– concentrations in groundwater 4 m below the water table in the presence of two and three upgradient disposal systems (results at 1000 d).

View larger version (12K): [in a new window]   Fig. 5. The effect of separation distance between two adjacent disposal systems on NO3– accumulation in groundwater (results at 1000 d, 4 m below the water table).

View larger version (16K): [in a new window]   Fig. 6. Comparison of NO3– concentrations in groundwater simulated in this study (at hydraulic conductivity K values of 600 and 1000 m d–1) and observed (Obs) in 1977 by Sinton (1982) for the nine wells that were sampled on the same dates (1 July and 1 Aug. 1977).

View larger version (9K): [in a new window]   Fig. 7. Simulated concentrations of fecal coliforms in groundwater 1 m below the water table (results at 1000 d).

View larger version (18K): [in a new window]   Fig. 8. Sensitivity of NO3– concentrations simulated using the range of parameter values for a specific location (distance 1907 m, 4 m below water table) and time (1000 d).

View larger version (37K): [in a new window]   Fig. 9. Sensitivity of NO3– concentrations simulated using a range of parameter values for the cross-section 4 m below the water table at 1000 d.

View larger version (20K): [in a new window]   Fig. 10. Simulated concentrations of fecal coliforms (on a logarithmic scale) at 1, 2, 3, and 4 m below the water table (mb.w.1) at a distance of 2005 m. The sensitivity analysis was performed with different removal rates in the unsaturated zone (results at 1000 d).