biological changes of nutrients in the samples. At times, 

 day-to-day changes of nitrogen concentrations in tile 

 drainage also had to be determined. The methods used to 

 determine nitrates, phosphates, and electrical conductivity, 

 and to prepajr'e soil extracts are discussed in this 

 subsection. 



Nitrogen Determinations . Nitrogen determinations of tile 

 drainage were made solely by the Department of Water 

 Resources' laboratory at Bryte, California, prior to 1966. 

 The Brucine method (11) is the accepted method used by the 

 DWR laboratory to determine nitrates. 



During this investigation most of the nitrogen determina- 

 tions were made in the San Joaquin District water quality 

 laboratory by the cadmium reduction method of nutrient 

 analysis, which was determined to be sufficiently accurate 

 for monitoring nutrients in tile drainage from the San 

 Joaquin Valley. This method relies on the conversion of 

 nitrate to nitrite. It is basically a colorimetric deter- 

 mination using a photoelectric cell and a fixed wave length 

 filter. This method was selected for two reasons: 

 (1) because immediate analyses of a large number of samples 

 could be made rapidly with an accurate account of the 

 nitrate-nitrogen concentrations, and (2) nitrates consti- 

 tuted more than 90 percent of the nitrogen in the tile 

 drainage. (Organic and ammoniacal nitrogen was found to be 

 less than 1 milligram per liter in tile drainage according 

 to Kjeldahl analyses made by the DWR laboratory.) 



The cadmium method was first used for determining the 

 general range of nutrients in the field. Accuracy and 

 precision were later improved, due to the availability of 

 better reagents and the development of a continuous standard- 

 ization and checking process. A "standard curve" was estab- 

 lished to maintain accuracy of the nitrate determinations 

 throughout the study. In order to do this, an array of 

 nitrate standards were obtained from the DWR laboratory that 

 ranged from 5 to 400 mg/l . Repeated tests were made on 

 these samples in order to determine the transmittance for 

 different standard concentrations . From these data a 

 "standard curve" was established to which samples containing 

 unknown concentrations could be con^ared. An average 

 standard deviation of less than 4.0 percent was achieved 

 time after time for a series of laboratory-prepared nitrate 

 standards that ranged from I5 to 28O mg/l. 



As part of the checking process, approximately 5 percent of 

 the field samples were split between the two methods of 

 analysis. A statistical comparison showed a correlation 

 coefficient of 0.992 for 77 split samples; other spot compar- 

 isons also showed a close correlation between the two methods 



37 



