FIGURE 9. SAMPLING METHODS — NUTRIENTS 



Collection of Mineral Samples . Mineral samples were 

 collected routinely during the initial investigations and 

 less frequently during the last few years of the study. 

 These samples were collected as back-up data for the nutrient 

 investigations and as a basis for determining other forms of 

 nitrogen and related constituents in the tile drainage. 



Collection of Dissolved Oxygen Samples . Effluent from 

 15 subsurface agricultural drainage systems was examined for 

 concentrations of dissolved oxygen (DO) to compare DO concen- 

 trations in drainage between tile systems in different major 

 tiled areas. Five systems were sampled in each of the 

 following major tiled areas: northern ( Byron-Gust ine), 

 central (Gustine-Mendota) and southern (Corcoran-Wasco) . 



Seimples were collected from tile outlets, tile discharge 

 sumps, gravity drain pools and pump discharge pipelines. A 

 long, rigid plastic tube was used to obtain samples from 

 inside tile outlets. 



Laboratory Techniques 



As many as 3OO samples per month were collected during I967 

 and 1968. Nitrate and phosphate determinations had to be 

 made as soon as possible after sampling to prevent possible 



36 



