Dissolved Oxygen Depletion in the Stockton Deep Water Ship Channel: Physical and Chemical Processes Conceptual Model

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Reach 2 DO and BOD Transported to Reach 3

Based on data collected in June–October 2001, Reach 2 BOD10 values at Channel Point ranged from approximately 6 to 10 mg/L. BOD5 values during this period ranged from 2 to 6 mg/L. At Channel Point, ebb tide BOD10 measurements were generally greater than BOD10 flood tide measurements. Ebb tide and flood tide BOD5 measurements were very similar (Lehman 2003).

DO measurements at Channel Point are affected by reaeration in the San Joaquin River and, because of the tides, DO concentrations upstream and DO concentrations in the DWSC. Upstream at Mossdale, DO measurements ranged from 7.6 to 12.2 mg/L and averaged 9.3 mg/L during June through October in 2001 (Jones & Stokes 2001a). At Channel Point, DO measurements ranged from 4 to 6.5 mg/L during June through September 2001 (Lehman 2003). DO measurements at Channel Point can decrease below the DO objectives (regulatory minimum), especially during flood tides that bring the lower DO concentrations from the DWSC past Channel Point.

How Reach 2 Dissolved Oxygen and Biochemical Oxygen Demand May Affect Dissolved Oxygen Concentrations in the Deep Water Ship Channel

Dissolved Oxygen

Because Reach 2 flows enter Reach 3 directly, the DO concentrations in Reach 2 would be expected to have more of an effect on DO concentrations in the DWSC than Reach 1 DO concentrations. Higher DO concentrations in Reach 2 will increase the initial DO concentrations in the DWSC and reduce the potential for the Reach 3 DO concentrations to drop below the DO objectives.

Biochemical Oxygen Demand

Diversions from Reach 2 (primarily at Old River) can remove large volumes of water and associated loads of algae and BOD and can therefore reduce the potential effects of algal biomass loads contributed from Reach 1 on dissolved oxygen concentrations in the DWSC (Quinn and Tulloch 2002). Consequently, when flow and algal biomass from Reach 1 are being diverted (primarily at Old River) the algal biomass generated in Reach 2 becomes more important.

Studies suggest that additional information is needed to determine the importance of algal loads from Reach 2 on the dissolved oxygen concentrations in the DWSC. Although data suggests that algal loads in the DWSC are derived from growth in the DWSC and from sources upstream (Lehman et al. 2001, Chen and Tsai 2002), it is uncertain how much of the algal load entering the DWSC from the San Joaquin River is from algae grown between Vernalis and Channel Point or from algal inputs contributed from Reach 1. It has been observed that concentrations and loads of suspended materials, including algae, decrease between Vernalis and Channel Point (Lehman 2003), which could indicate that Reach 1 BOD loads are less important to DWSC dissolved oxygen concentrations than BOD loads originated from Reach 2. This is supported by studies that found that algal species in Reach 2 are similar to those in the DWSC (Van Nieuwenhuyse 2002) and that algal species measured upstream of Vernalis is different from the species measured downstream of Vernalis and in the DWSC (Lehman 2003). One reason for the uncertainty about the importance of the Reach 2 BOD loads is that the algal biomass dynamics in the San Joaquin River, including the potential settling and decay of algae and organic materials in Reach 2, are not well understood (Jones & Stokes 2001a, Stringfellow and McGahan 2003).

Studies suggest that the Reach 2 BOD loads may be a significant factor affecting the dissolved oxygen concentrations of the DWSC. A good indication that the RWCF is a significant source of NBOD is that when effluent from the RWCF contains high ammonia concentrations, the percentage of NBOD in the DWSC is higher (Lee and Jones-Lee 2003, Lehman et al. 2001, Lehman 2003, Hunt 2002). The poor association between the organic nitrogen load from upstream and both dissolved ammonia concentration and NBOD was confirmed by correlation analysis and trend plots (Lehman 2003), and may support the significance of ammonia loads from the RWCF.

The ammonia load from upstream of the RWCF at Mossdale was only about 10 percent of the RWCF load, but it could make a more substantial contribution to oxygen demand if ammonia increases during passage from Mossdale to the DWSC. The San Joaquin Drainage Authority is conducting experiments to determine more accurately the effects of the ammonia in this San Joaquin River reach (2) and determine the importance of BOD loads from Reach 1 compared to BOD loads from Reach 2 on DWSC DO concentrations. Understanding and predicting how fast organic nitrogen in decaying and how rapidly ammonia is oxidized in this region is important to assigning the oxygen demand allocation between algal biomass and ammonia (Stringfellow and McGahan 2003).

Measurements of total BOD5 and CBOD5 suggest that algal biomass is an important contributor to BOD concentrations in Reach 2 and ammonia may be a less significant contributor to BOD concentrations in Reach 2 than algal inputs. The importance of imported BOD concentrations to the BOD concentration in Reach 2 is uncertain.