Sources of Oxygen Demand in the Lower San Joaquin River, California P. W. Lehman, J. Sevier, J. Giulianotti & M. Johnson California Department of Water Resources

Sacramento Stockton San Francisco Bay Antioch Mossdale CALIFORNIA 38 20’ W River o e m River c r a a S San Stockton 38 00’ W Antioch Carquinez straight Mossdale San Francisco Bay

Current situation: There has been little change in dissolved oxygen concentration in the channel despite enhanced management and a decrease in phytoplankton biomass

Frequency of values below 5 mg l-1

Chlorophyll a decreased over time in the channel

Question: What are the primary sources of oxygen demand in the Deep Water Channel near Stockton?

Study Methods biweekly or monthly sampling July - November 2000 & 2001 discrete variables: Primary productivity : in situ dissolved oxygen light and dark bottle technique Nutrient concentrations chlorophyll a and phaeophytin concentration BOD tests continuous variables vertical profiles with YSI 6600 sonde continuous flow continuous water quality

MD TB CP TC RR L48 San Joaquin River Deep Water Channel VN TB CP TC RR STOCKTON MC DONALD MD ROBERTS ISLAND Rough and Ready Island Calaveras River TB CP TC RR L48 San Joaquin River Deep Water Channel N 4 km Middle River VN 80 o 00 ’ W 121 20 Water Treatment Plant TC Turner Cut RR Rough and Ready Island L48 Navigation Light 48 TB Turning Basin CP Channel Point MD Mossdale VN Vernalis Island Calaveras River TB CP TC RR L48 San Joaquin River Deep Water Channel N 4 km 121 o 20 ’ N 80 o 00 ’ W Water Treatment Plant Middle River N N 4 km VN

Findings

Dissolved oxygen in DWSC

Dissolved oxygen upstream

Oxygen depletion was not caused by stratification

Oxygen demand was not caused by phytoplankton respiration

Oxygen demand was caused by nitrification

Pearson Correlation Coefficients (n=103) TBOD & NBOD 0.86 NBOD & ammonia 0.93 NBOD & organic N 0.34 TBOD & ammonia 0.78 TBOD & CBOD 0.62 TBOD & chlorophyll 0.59

Stepwise Regression Dissolved ammonia + carbonaceous BOD 60% 30% Year n R2 2000 100 0.91 2001 85 0.73 2000 + 2001 185 0.83

Where does the ammonia come from ?

NBOD was correlated with ammonia load from treatment plant

NBOD at Rough and Ready varied with the ammonia load from the treatment plant

Total nitrogen load was higher from upstream

Mass Balance Model Object: Determine the largest source of dissolved ammonia in the channel

Simple mass balance model Daily dissolved ammonia load into the ship channel from the treatment plant and upstream = 1) daily load of dissolved ammonia from each source + 2) daily load of dissolved ammonia from the oxidation of the organic nitrogen load from each source + 3) daily load of dissolved ammonia from the oxidation of residual organic nitrogen from previous day from each source

Ammonification rate

Upper boundary condition: all organic nitrogen oxidized at ammonification rate for chlorophyll (highest rate)

Chlorophyll was a small percent of the organic nitrogen load

Lower boundary condition: only the organic matter associated with live chlorophyll was oxidized

Summary Oxygen demand in the channel was primarily caused by nitrification The treatment plant could be the primary cause of nitrification in the channel on a daily basis even though it had a small ammonia load because it was a direct source of dissolved ammonia for bacterial oxidation The relative contribution of ammonia from the treatment plant and upstream to ammonia in the channel was a function of residence time, ammonification rate, direct loads and load composition

Take home message The oxygen demand produced from the direct load of dissolved ammonia from the treatment plant could have a greater impact on daily oxygen demand in the channel than the oxidation of organic nitrogen from upstream because of the slow oxidation rate and low reactivity of upstream organic matter

Net transport of phytoplankton mass decreased downstream