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

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Reach 1 DO and BOD Transported to Reach 2

The DO concentration and the BOD concentration at the downstream end of Reach 1 (Vernalis) is transferred with the river flow in Reach 2. The daily load of DO or BOD can be estimated as:

daily load (lbs/day) = 5.4 x flow (cfs) x concentration (mg/L)

For example, if the DO saturation is 8 mg/L and the flow is 1,000 cfs, a daily load of 43,200 lb/day of oxygen is transported into Reach 2. The BOD concentration may require 30 days to fully decay. The San Joaquin River carries a very large load of both DO and BOD. Both must be considered in the evaluation of DO in the river.

How Reach 1 Dissolved Oxygen and Biochemical Oxygen Demand May Affect Dissolved Oxygen Concentrations in the Deep Water Ship Channel

Dissolved Oxygen

Much of the DO concentration from Reach 1 may be transferred downstream to the DWSC (Reach 3). The residence time in Reach 2 is between 64 hours at 750 cfs and 12 hours at 7,000 cfs.

Biochemical Oxygen Demand

One study found that the DWSC received more CBOD5 from river loads (at Vernalis) than from the Stockton RWCF (Jones & Stokes 2003), which may indicate that Reach 1 BOD sources affect DO concentrations in the DWSC.

Differences in algal species measured upstream of Vernalis and the algal species measured in the DWSC indicate that algal species changed or were replaced as they moved downstream in the San Joaquin River (Lehman 2003). These results indicate that algal biomass upstream of Vernalis may not significantly affect the DO concentrations in the DWSC because the majority of algae in the DWSC may have grown in the DWSC. (Lehman 2003.)

Other studies found that algal biomass from Reach 1 (measured at Vernalis) was greater than the biomass measured near the San Joaquin River and DWSC confluence (Lehman et al. 2001). This difference indicates vertical settling, benthic and planktonic grazing, or other factors may be removing Reach 1–derived algae in Reach 2 and thereby reducing the effect of Reach 1 algae contributions on DO concentrations in the DWSC (Lehman et al. 2001).

High algal concentrations and a long relative travel time (approximately 2–4 days) from Vernalis to the DWSC affect the level of oxygen demand and oxygen-demanding substances reaching the DWSC from Reach 1 (Foe et al. 2002). At lower flows in the DWSC, the residence time increases and the relative contribution of Reach 1 NBOD loads may increase (Lehman 2003). The increased residence time may allow more organic nitrogen (from algae) to be converted to ammonia (Lehman 2003), but nitrification (ammonia oxidizing to nitrate) in Reach 2 will reduce the NBOD. In general, the Stockton RWCF discharged greater daily dissolved ammonia loads than upstream of Vernalis (Lehman et al. 2004). However, the Reach 1 algal loads strongly affected the relative contribution of the Stockton RWCF and Reach 1 to the dissolved ammonia loads in the DWSC (Lehman et al. 2004). Thus, the importance of Reach 1 NBOD loads depends on algal loads upstream of Vernalis.

BOD concentrations transported downstream to Reach 1 may increase oxygen demand loads in the DWSC. However, flows reaching the DWSC from Reach 1 may reduce the hydraulic residence time in the DWSC, reducing the time that the BOD transported from Reach 1 is exerted in the DWSC. In addition, higher flows dilute the concentration of BOD and ammonia from the RWCF. Diversions from the San Joaquin River in Reach 2 (i.e., Old River) can remove large volumes of algal biomass and therefore reduce the potential effects of Reach 1 BOD loads on DO concentrations in the DWSC (Quinn and Tulloch 2002).