Scoping study of transient TDS/EC conversion ratios in the San Joaquin River Basin
The salinity of surface and groundwater water bodies is determined by summing the mass of the various dissolved anions and cations - salinity is typically measured in parts per thousand or milligrams per liter and determined gravimetrically in the Laboratory or by a solid state instrument such as a mass spectometer. Gravimetric analysis of salinity samples is tedious and time consuming; mass spectrometry is expensive and limited to laboratories that can maintain specialized instruments. In the field, the use of electronic sensors that measure the conductance of dissolved solids in water directly, and that rely on stable relationships between the mass of ions in solution (TDS)and conductance readings (EC), has been widespread. Electrical conductivity (EC) sondes have become the most ubiqitous water quality sensors in commercial use. The stability of the TDS/EC relationship, however is not commonly tested - the assumption that the ratio remains fixed seasonally or annually may be invalid.
Over the past decade the TDS/EC ratio in wetland channels on the west-side of the San Joaquin Valley appears to have drifted from a high of around 0.74 during the mid 1980's, when selenium contamination at Kesterson Reservoir unleashed a tidal wave of environmental monitoring, to a low of 0.61 today. These ratios were determined mostly from gravimetric analysis in the laboratory and not from total ion counts. The significance of this drift is high and has implications for salinity mass loading assessment under the current salinity TMDL in the Basin. This leads to the following research questions :
1. What is the reason for this drift?
2. Has there been a major shift in return flow ion composition over the past 15 years?
3. Is there a stable relationship between salinity concentration estimates made by gravimetric analysis and by ion summation?
4. Is the current ratio of 0.61 valid for wetland return flows year-round?
5. What are the Reclamation region-wide implications?
Need and Benefit
Reclamation has a responsibility to provide drainage service to its water contractors. The accounting of Reclamation's responsibility under the current salinity TMDL is related to the salt loads in the water supply diverted from the Delta Mendota Canal to Reclamation contractors on the west side of the San Joaquin Valley. Measurement of these salt loads is made from electrical conductivity (EC) readings made using solid state salinity sensors (typically a Wheatstone resistance measurement) and applying a fixed TDS/EC (ratio)factor to estimate total dissolved solids. These factors are assumed stable and many of these instruments currently deployed within the watershed allow these factors to be entered once as a fixed input parameter before sonde deployment. This resaerch project calls into question this practice - especially given the trend in declining EC values measured in wetland channels in the Grasslands Ecological Area on the west-side of the San Joaquin Basin.
This research focuses on the salinity TMDL applied to managed wetlands in the San Joaquin Basin for the major reason that there has been more data collected for this region than other regions on TDS/EC ratios. This is also the region that is attempting the first implementation of real-time salinity management - hence this ratio is critical to the scientific basis for implementation. The need is therefore focused on decisions wetland managers will need to make relative to the scheduling of drainage drawdown and the relationship between these salt loads and the assimilative capacity of the San Joaquin River.
The benefit of an appropriately designed and fully-functional decision support system will be an enhanced capability for wetland managers to coordinate and schedule saline return flows to the San Joaquin River and to improve compliance with State regulations for salinity. The Central Valley Regional Water Quality Control Board granted an excursion to the strict TMDL salt load discharge allocation methodology that would go into effect if Reclamation and its contractors collectively implemented a real-time salinity management system. It is critical that the salinity loading calculated from this subarea is correctly calculated with current TDS/EC ratios.
A scoping document will be submitted for public viewing.