Increased since 1966. Tile drainage discharges for future 

 periods from these and other aireas have been predicted by 

 the San Joaquin Valley Drainage Advisory Group (30) . 



San Luis Unit Service Area 



One of the most Important areas to consider from the stand- 

 point of future drainage is the Federal San Luis Unit Service 

 Area. The U. S. Bureau of Reclamation estimates that the 

 annual agricultural wastewater disposal requirement (30) 

 will reach 155*000 acre-feet in 50 years and is expected to 

 exceed this by 2035- This amounts to better than one-third 

 the total volume predicted from the entire San Joaquin 

 Valley for the same period. 



An important consideration, aside from the quantity of 

 drainage from this area, is the expected nitrogen content. 

 If the drainage is high in nitrogen, it could raise the 

 predicted levels higher than anticipated and could also 

 affect the seasonal variability of nutrient concentrations. 

 Soils within the SLUSA are among those already mentioned: 

 Panoche, Panhlll, Lost Hills, Oxalls, Levis, and Lethent . 

 However, according to the tile drainage plans proposed by 

 Westlands Water District, the largest amount of the 300,000 

 acres that are to be drained will be located along the 

 eastern portion of the area near the trough, which is 

 represented mostly by the Oxalls soil series of the basin 

 rim physiographic position. The Lethent and Levis series 

 also occupy sizable acreages In the area. There also may be 

 some Influence from Panoche soils where tile system laterals 

 extend into the lower alluvial fan position. 



According to findings mentioned in previous sections of this 

 report, tile drainage from basin rim and basin position 

 soils in the nearby Gustlne-Mendota area was never found to 

 be high in nitrogen. Moderate concentrations were observed 

 in a few tile drains where soils of recent alluvial fans 

 merge with soils of the basin rim position. Therefore, tile 

 drainage data would seem to indicate that nitrogen concentra- 

 tions from the basin rim soils in the San Luis Unit Service 

 Area would be lower than 33 mg/l ^ the I967-68 average for 

 the Gustlne-Mendota area. This area consists mostly of 

 soils occupying recent alluvial fans. 



However, in some cases basin rim soils appear to contain 

 high nitrates. Investigations by the Department have shown 

 that residual nitrogen in soils of the basin rim was quite 

 low, except for two sites of Lethent soils located in the 

 north end of the SLUSA. The USER found sites of high 



80 



