Continuous Water Quality Report

Author

Andrew Tran, Vivian Klotz, Catarina Pien

Introduction

The Department of Water Resources (DWR) and the US Bureau of Reclamation (USBR) are required by Water Right Decision 1641 (D-1641) to collect continuous water chemistry data to monitor the water quality at select sites in the upper San Francisco Estuary (Estuary). This report describes the results of these monitoring efforts for water year 2024 (October 1st 2023 through September 30th 2024) which was classified as above normal in the Sacramento and San Joaquin Valleys (source).

This report includes mandated stations collected by DWR’s Continuous Environmental Monitoring Program (CEMP) and Suisun Marsh Water Quality Program (SMWQP) as well as USBR’s Delta Water Quality Monitoring Program (USBR). Stations monitored by other programs were not included in this report. Additional information about these programs can be found here.

Methods

Water quality data was continuously monitored at 29 sites throughout the Upper Estuary and were grouped into regions based on their geographic location (Figure 1; Table 1). These sites represent a variety of aquatic habitats, from narrow, freshwater channels to broad, estuarine bays.

Notes:

  • The Sherman Island station (SSI), which was called D22A in previous reports, has been changed back to D11A.

  • The Cache Slough station (C19) is listed as a monitoring station for D-1641 but has not been in operation since 2015 due to lack of use and safety concerns.

Figure 1: Map of D-1641 continuous WQ field sites
Table 1: Continuous WQ stations included within each region of the Delta
Region WY Index Stations
Central Delta San Joaquin C4, C5, D16A, D19A, D29
Confluence Sacramento C2, D9A, D11A, D12A, D22
North Delta Sacramento C3A, D24A
South Delta San Joaquin C7A, C8, C10A, DMC1, P8A
Suisun and Grizzly Bays Sacramento C14, D6A, D7A, D8A, D10A
Suisun Marsh Sacramento S21, S33, S35, S42, S49, S64, S71

Data were collected for six water quality parameters and stations were divided between programs (Table 2). For CEMP and SMWQP, all water samples were collected 1-meter below the water surface using a float-mounted YSI EXO2 multi-parameter water quality sonde. These parameters are:

  • Water Temperature (°C)
  • Specific Conductance (μS/cm 25°C)
  • Dissolved Oxygen (mg/L)
  • pH
  • Turbidity (FNU)
  • Fluorescence (μg/L)

For Reclamation, water samples were collected 1-meter below the water surface using float-mounted Hydrolab MS5 and HL4 water quality sondes. Parameters collected include:

  • Water Temperature (°C)
  • Specific Conductance (μS/cm 25°C)
Table 2: List of device types used at continuous WQ stations per program
Program Stations
DWR-EMP C2, C3A, C7A, C10A, D6A, D7A, D8A, D9A, D10A, D11A, D12A, D16A, D19A, D24A, D29, P8
DWR-SMWQP S21, S33, S35, S42, S49, S64, S71
USBR-DWQMP C2, C4, C5, C8, C14, D22, DMC1

Regional facet graphs were created for each parameter. The average, minimum, and maximum values were determined for parameter, both overall and per region. Average summary statistics are reported as the mean (μ) ± the standard deviation.

For more in-depth methodology, see here.

Results

Water Temperature

The average water temperature value was 17.10 ± 4.66 °C. Values ranged from 7.34 °C to 29.10 °C. Per region average, minimum, and maximum values are shown in Table 3; time series plots are shown in Figure 2.

Table 3: Summary statistics for water temperature by region in the San Francisco Bay-Delta estuary.
Statistic Central Delta Confluence North Delta South Delta Suisun and Grizzly Bays Suisun Marsh
Average 17.40 16.70 16.30 17.90 16.70 17.10
Min 9.33 9.28 8.65 8.76 9.82 7.34
Max 26.60 24.60 24.30 29.10 24.40 26.00
Figure 2: Water temperature by region in the San Francisco Bay-Delta estuary.

Specific Conductance

The average specific conductance value was 2990 ± 4790 µS/cm. Values ranged from 50 µS/cm to 28700 µS/cm. Per region average, minimum, and maximum values are shown in Table 4; time series plots are shown in Figure 3.

Table 4: Summary statistics for specific conductance by region in the San Francisco Bay-Delta estuary.
Statistic Central Delta Confluence North Delta South Delta Suisun and Grizzly Bays Suisun Marsh
Average 247 2260 153 330 9110 4630
Min 64 85 98 50 110 213
Max 1300 16300 294 785 28700 16700
Figure 3: Specific conductance by region in the San Francisco Bay-Delta estuary.

Dissolved Oxygen

The average dissolved oxygen value was 8.70 ± 1.22 mg/L. Values ranged from 2.67 mg/L to 15.30 mg/L. Per region average, minimum, and maximum values are shown in Table 5; time series plots are shown in Figure 4.

Table 5: Summary statistics for dissolved oxygen by region in the San Francisco Bay-Delta estuary.
Statistic Central Delta Confluence North Delta South Delta Suisun and Grizzly Bays Suisun Marsh
Average 9.31 9.10 9.23 8.98 8.95 7.46
Min 7.65 7.64 7.60 4.91 7.59 2.67
Max 11.80 10.60 11.00 15.30 10.50 10.10
Figure 4: Dissolved oxygen by region in the San Francisco Bay-Delta estuary.

Stockton Station DO Values

C-EMP monitors DO at the Stockton Ship channel to determine if/when it fall below limits established by the CVRWQCB (1998). For the months that coincide with the passage of fall-run Chinook salmon (October, November, and September), values fell below the 6 mg/L limit in September. For all other months, values fell below the 5 mg/L limit in July. A boxplot of the DO values is shown in Figure 5.

Figure 5: Range of daily dissolved oxygen values at the Stockton Ship Channel (P8).

pH

The average pH value was 7.68 ± 0.25. Values ranged from 7.16 to 9.31. Per region average, minimum, and maximum values are shown in Table 6; time series plots are shown in Figure 6.

Table 6: Summary statistics for pH by region in the San Francisco Bay-Delta estuary.
Statistic Central Delta Confluence North Delta South Delta Suisun and Grizzly Bays Suisun Marsh
Average 7.80 7.75 7.59 7.65 7.82 7.50
Min 7.22 7.41 7.22 7.18 7.50 7.16
Max 9.31 8.18 7.94 9.15 8.37 7.86
Figure 6: pH by region in the San Francisco Bay-Delta estuary.

Turbidity

The average turbidity value was 20.10 ± 18.40 FNU. Values ranged from 0.17 FNU to 178.00 FNU. Per region average, minimum, and maximum values are shown in Table 7; time series plots are shown in Figure 7.

Table 7: Summary statistics for turbidity by region in the San Francisco Bay-Delta estuary.
Statistic Central Delta Confluence North Delta South Delta Suisun and Grizzly Bays Suisun Marsh
Average 5.24 16.70 12.80 11.30 30.10 37.40
Min 0.17 4.44 2.47 2.15 5.79 9.01
Max 47.70 82.70 119.00 126.00 178.00 127.00
Figure 7: Turbidity by region in the San Francisco Bay-Delta estuary.

Chlorophyll a Fluorescence

The average fluorescence value was 3.72 ± 5.16 µg/L. Values ranged from 0.72 µg/L to 87.90 µg/L. Per region average, minimum, and maximum values are shown in Table 8; time series plots are shown in Figure 8.

Table 8: Summary statistics for chlorophyll a fluorescence by region in the San Francisco Bay-Delta estuary.
Statistic Central Delta Confluence North Delta South Delta Suisun and Grizzly Bays Suisun Marsh
Average 2.23 2.24 2.30 8.01 2.90 4.42
Min 0.73 0.72 0.74 1.30 0.95 1.10
Max 5.33 10.00 6.47 87.90 13.70 16.90
Figure 8: Chlorophyll a fluorescence by region in the San Francisco Bay-Delta estuary.

Interpretations

Water Year 2024 started off dry, but a series of storms in February and March brought the water year to be classified as Above Normal. Patterns in water quality generally followed expected seasonal and regional trends. Specific conductivity varied greatly across the stations and throughout the water year, depending on precipitation and Delta outflow. North and South Delta stations remained fresh throughout the year, whereas most Confluence, Suisun Marsh, and Suisun and Grizzly Bay stations had low specific conductivity values from February through early June. The specific conductivity at D6A, the westernmost station, started increasing in mid-March, far earlier than the rest of the stations. Decreases in dissolved oxygen and increases in turbidity were observed between February and March, likely associated with increased discharge from storm events. Shallow bays and Suisun Marsh exhibited more persistently high and variable turbidity compared with other stations. Elevated fluorescence, dissolved oxygen, and pH were measured in the South Delta during late June and July associated with a bloom event.

References

[CVRWQCB] Central Valley Regional Water Quality Control Board. (1998). Water Quality Control Plan for the California Regional Water Quality Control Board Central Valley Region, the Sacramento River Basin, and San Joaquin River Basin [Basin Plan] (4th ed.).

[SWRCB] State Water Resources Control Board. (1995). Water Quality Control Plan for the San Francisco Bay/Sacramento-San Joaquin Estuary [Bay-Delta Plan] (Adopted May 22, 1995, pursuant to Water Right Order 95-1). Sacramento, CA.

[SWRCB] State Water Resources Control Board. (1999). Water Rights Decision 1641 for the Sacramento-San Joaquin Delta and Suisun Marsh (Adopted December 29, 1999, Revised in Accordance with order WR2000-02 March 15, 2000). Sacramento, CA.

Archived Reports

Old EMP continuous water quality reports can be found here.