Total dissolved solids?

[Hartman, Rosemary@DWR (she/her)]

Hi Folks,

 

Has anyone here done work with total dissolved solids data? Here at DWR, we are standardizing our analytical methods for TDS, and trying to figure out the impact of the method change on our data. Katey (cc’d) is heading up the effort, and asked me how big of a difference is meaningful for someone analyzing the data. I have honestly never analyzed TDS data for anything before, so I’m asking the bigger group. Does anyone have advice on how big of a difference in TDS between methods would be ecologically meaningful?

 

Thanks in advance for any help!

 

---
Rosemary Hartman, PhD, she/her
Environmental Program Manager

Rosemary...@water.ca.gov
California Department of Water Resources
M: 916-882-2926

saveourwater.com

 

saveourwater.com

 

[Flynn, Ted@DWR (he/him)]

I’ve worked with TDS data some. One difference between methods could certainly be choice of filter. If two methods differed in the pore size of filter used, that would certainly bias the data in a particular direction. Drying temperature might also make a difference (higher temps could cause some minerals to dehydrate or otherwise break down, giving a different mass).

 

It would probably depend on how you define “ecologically meaningful.” In my experience, TDS is primarily used a measurement of aesthetic water quality rather than something tied to, say, a particular biogeochemical function in an ecosystem. Did you have a particular usage case in mind?

 

-Ted

 

Ted Flynn, PhD Unit Manager, DWR-SWP-DISE Discrete Environmental Monitoring West Sacramento, CA +1 916 820 5052 | saveourwater.com

 

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[Rein, Katheryn (Katey)@DWR (she/her)]

Hey Ted,

 

Thanks for your thoughts! We’re going to be looking at differences between our department’s more historical processing of TDS samples compared to some upcoming changes to come into compliance with the analytical method (Standard Methods 2540D). I’m aiming to be able to implement this study to cover the range of concentrations found across the delta and other regions our departmental programs sample across, but it would be helpful to know if there are TDS concentration ranges (e.g., <50 mg/L, <100 mg/L, >1000 mg/L) that have ecological significance so I can best bracket these ranges.

 

I’m in the office today and happy to chat further too 😊

 

Kindly,

 

Katey Rein (She/Her)

Environmental Scientist, QA Program

 

Division of Integrated Science and Engineering

Department of Water Resources

916-203-3323

[Peter Nelson]

This response avoids the (good) question about what's "ecologically meaningful", but, if there are data available using both methods applied to the same samples (ie a range of TDLs), you could compare the measurement variance obtained using the two methods. If the variance associated with the new method is the same or lower than what you found with the first, you'd have a good argument that, whatever might be ecologically meaningful, the new method is better!

I'm sure you've thought of this, but what's ecologically meaningful to one org might be absolutely no big deal to another...

Cheers, Pete

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[Swift, Ted@DWR (he/his)]

There are several questions woven together here.

As I understand it, TDS is most often used in municipal water quality regulatory standards (often including specific ions), while SC ("EC") is most often used for environmental monitoring, ecology, and the convenience of real-time "quick look" monitoring. Bryte Lab does analyses for both SC and TDS, and is changing their method of analyzing for TDS.

Total dissolved solids (TDS) is very highly correlated with SC, with R^2 ~0.95 - 0.999 (Though it's really SC, it's very often written as "EC"). 

Ecologically, certain species have different tolerances, or thresholds, for salinity (usually expressed as SC). These tolerances vary by species, of course. I think to answer Katey's original question, perhaps we can assemble some example EC tolerances, what change would be ecologically meaningful, and convert those SCs to TDS (and/or TDS differences).

TL, but useful background:

Paul Hutton and Sujoy Roy reviewed the relationships between TDS, SC and individual ions in

Extension of the Practical Salinity Scale to Estimate Major Ion Concentrations: Application to the San Francisco Estuary | Estuaries and Coasts . They stood on the shoulders of Denton (2015, ref below).

They wrote "The estuary’s salinity gradient is regulated through management of water discharges from reservoirs and freshwater withdrawals for agricultural and municipal use (CSWRCB 1999). While contemporary management has typically focused on total salinity, expressed in the form of specific electrical conductance (EC), interest in the concentrations of total dissolved solids (TDS) and individual major ions also exists, as these measures are related to regulatory standards and potential uses of these waters for municipal supply (Denton 2015). Empirical quadratic relationships between EC, total dissolved solids, and major ions have been developed for different regions of the estuary to inform these interests (Denton, R.A. 2015. Delta salinity constituent analysis, report prepared for the State Water Contractors Authority, Feb 2015. Available online at: https:// rtdf.info/ [link seems to be broken ).

New link to Denton (2015) can be found at Denton_2015_Delta_Salinity_Constituents_Report.pdf

Denton (2015) is pretty long and takes some wading through. The relationship(s) between SC ("EC") and TDS is very tight and have been summarized and reviewed in Estimating Conservative Constituent Concentrations Across the Sacramento-San Joaquin River Delta (see SC vs TDS regression values in, e.g., Tables 2 and 3). The intended audience is mostly modelers, but that's OK.

-Ted S "the other Ted"

__________________________________________

Ted Swift, Ph.D. (He, Him, His), Senior Environmental Scientist (Specialist),

Quality Assurance Program, Division of Integrated Science and Engineering

Department of Water Resources; New phone number: 279/789-4554

“Reality must take precedence over public relations, for Nature cannot be fooled.”

    -Richard Feynman

 

---

From: bay-delta-...@googlegroups.com <bay-delta-...@googlegroups.com> on behalf of Peter Nelson <p.ne...@gmail.com>
Sent: Wednesday, February 5, 2025 4:21 PM
To: Rein, Katheryn (Katey)@DWR (she/her) <Kather...@water.ca.gov>
Cc: Flynn, Ted@DWR (he/him) <Ted....@water.ca.gov>; Hartman, Rosemary@DWR (she/her) <Rosemary...@water.ca.gov>; Bay-Delta Data Science Fun <bay-delta-...@googlegroups.com>
Subject: Re: [bay-delta-datascience] RE: Total dissolved solids?

 

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[Hartman, Rosemary@DWR (she/her)]

Some additional context might be helpful:

• We are changing the method to better comply with certification requirements. So it’s not about which method is “better”.
• What we need to know is whether the old method provides significantly different results from the new method. So, is it biased high or low, or it its variance higher or lower.
• We want to know what is “ecologically/management relevant” because we need to know what effect size we are looking for when designing a study to compare the two methods.
• For example, if we were comparing two specific conductance methods, one might be biased high by about 2 uS/cm, but no one would care. It wouldn’t be worth our time to collect enough samples to detect that very small effect size. In contrast, if it was off by 2000 uS/cm, you’d care.
• But I don’t know what effect size is relevant for TDS! If it’s used more for water quality than ecology, what effect size is relevant there?

 

 

Rosie

 

From: Swift, Ted@DWR (he/his) <Ted....@water.ca.gov>
Sent: Wednesday, February 5, 2025 5:51 PM
To: Peter Nelson <p.ne...@gmail.com>; Rein, Katheryn (Katey)@DWR (she/her) <Kather...@water.ca.gov>
Cc: Flynn, Ted@DWR (he/him) <Ted....@water.ca.gov>; Hartman, Rosemary@DWR (she/her) <Rosemary...@water.ca.gov>; Bay-Delta Data Science Fun <bay-delta-...@googlegroups.com>
Subject: Re: [bay-delta-datascience] RE: Total dissolved solids?

 

There are several questions woven together here.

As I understand it, TDS is most often used in municipal water quality regulatory standards (often including specific ions), while SC ("EC") is most often used for environmental monitoring, ecology, and the convenience of real-time "quick look" monitoring. Bryte Lab does analyses for both SC and TDS, and is changing their method of analyzing for TDS.

 

Total dissolved solids (TDS) is very highly correlated with SC, with R^2 ~0.95 - 0.999 (Though it's really SC, it's very often written as "EC"). 

 

Ecologically, certain species have different tolerances, or thresholds, for salinity (usually expressed as SC). These tolerances vary by species, of course. I think to answer Katey's original question, perhaps we can assemble some example EC tolerances, what change would be ecologically meaningful, and convert those SCs to TDS (and/or TDS differences).

 

 

TL, but useful background:

Paul Hutton and Sujoy Roy reviewed the relationships between TDS, SC and individual ions in

Extension of the Practical Salinity Scale to Estimate Major Ion Concentrations: Application to the San Francisco Estuary | Estuaries and Coasts . They stood on the shoulders of Denton (2015, ref below).

 

They wrote "The estuary’s salinity gradient is regulated through management of water discharges from reservoirs and freshwater withdrawals for agricultural and municipal use (CSWRCB 1999). While contemporary management has typically focused on total salinity, expressed in the form of specific electrical conductance (EC), interest in the concentrations of total dissolved solids (TDS) and individual major ions also exists, as these measures are related to regulatory standards and potential uses of these waters for municipal supply (Denton 2015). Empirical quadratic relationships between EC, total dissolved solids, and major ions have been developed for different regions of the estuary to inform these interests (Denton, R.A. 2015. Delta salinity constituent analysis, report prepared for the State Water Contractors Authority, Feb 2015. Available online at: https:// rtdf.info/ [link seems to be broken ).

New link to Denton (2015) can be found at Denton_2015_Delta_Salinity_Constituents_Report.pdf

 

Denton (2015) is pretty long and takes some wading through. The relationship(s) between SC ("EC") and TDS is very tight and have been summarized and reviewed in Estimating Conservative Constituent Concentrations Across the Sacramento-San Joaquin River Delta (see SC vs TDS regression values in, e.g., Tables 2 and 3). The intended audience is mostly modelers, but that's OK.

 

-Ted S "the other Ted"

__________________________________________

Ted Swift, Ph.D. (He, Him, His), Senior Environmental Scientist (Specialist),

Quality Assurance Program, Division of Integrated Science and Engineering

Department of Water Resources; New phone number: 279/789-4554

“Reality must take precedence over public relations, for Nature cannot be fooled.”

    -Richard Feynman

 

[Swift, Ted@DWR (he/his)]

Great points, Rosie,

Taking the conversion equations can help us map EC (SC) effect sizes to TDS for the ecological questions. But / and it's the municipal regulations that will inform what's a "significant difference" for TDS for regulatory reporting.

-Ted S

__________________________________________

Ted Swift, Ph.D. (He, Him, His), Senior Environmental Scientist (Specialist),

Quality Assurance Program, Division of Integrated Science and Engineering

Department of Water Resources; New phone number: 279/789-4554

“Reality must take precedence over public relations, for Nature cannot be fooled.”

    -Richard Feynman

 

---

From: Hartman, Rosemary@DWR (she/her) <Rosemary...@water.ca.gov>
Sent: Thursday, February 6, 2025 7:58 AM
To: Swift, Ted@DWR (he/his) <Ted....@water.ca.gov>; Peter Nelson <p.ne...@gmail.com>; Rein, Katheryn (Katey)@DWR (she/her) <Kather...@water.ca.gov>
Cc: Flynn, Ted@DWR (he/him) <Ted....@water.ca.gov>; Bay-Delta Data Science Fun <bay-delta-...@googlegroups.com>
Subject: RE: [bay-delta-datascience] RE: Total dissolved solids?