1. Changes in DOC Concentration, Composition, and Reactivity During Passage Through Constructed Wetlands of the Central Delta: Implications for Drinking Water Quality
Bay Delta Science Conference 2016
Angela Hansen*1, Tamara Kraus1, Elizabeth Stumpner1, Sandra Bachand2, William Horwath3, Phillip Bachand2
1USGS California Water Science Center, 6000 J Street, Sacramento, CA 95819, 2 Bachand and Associates, Davis, CA 95616, 3 University of California, Davis, One Shields Avenue, Davis, CA 95616
Overview
Results – DOC concentration and DBPFP
Optical Properties
Results - DBPFP Predictors
Wetland restoration in the Central Delta would not only provide habitat benefits to fish and wildlife, but would help ameliorate and potentially reverse subsidence. However, wetlands have the potential to add dissolved organic carbon (DOC) to Delta waterways which can negatively impact downstream drinking water quality because a fraction of this DOC pool reacts upon disinfection to form harmful disinfection byproducts (DBPs). In the Central Delta, water pumped off the subsided islands is already high in DOC due to inputs from organic peat soils.
This study examined whether in situ coagulation either alone or in conjunction with constructed wetlands can decrease export of DOC and DBP precursors from agricultural drainage waters exported from Twitchell Island. We used optical properties (absorbance and fluorescence) to investigate the chemical composition of the DOC to gain insight into its likely source (i.e. peat soils, plants, algae). We then related this to the reactivity of the DOC pool with respect to trihalomethane and haloacetic acid formation potentials (THMFP and HAAFP, respectively).
Optical measurements (absorbance and fluorescence) provide extensive information about the concentration and composition of DOM and its likely origin.
DOC concentrations in the untreated drainage water. (Co-In) were higher during winter months.
Coagulation alone reduced DOC by 65-85% over the ten-month study period.
Passage through wetlands often ADDED DOC for Co and Fe cells, especially summer (Mar-Oct).
STHMFP µmol HAA/mol DOC
SHAAFP µmol HAA/mol DOC
Even though DOC concentration changed, there was little change in DOM reactivity with respect to THMFP.
Coagulation treatment alone slightly decreased reactivity.
Wetland passage had little to no effect on STHMFP.
There were notable changes in reactivity with respect to HAAFP.
Coagulation preferentially removed HAA precursors.
Wetland passage in summer months increased SHAAFP, especially in the Fe treatments.
Common parameters and indices derived from optical data include the absorbance at specific wavelengths (e.g., A254, A280, A370), and fluorescence at specific excitation/emission pairs (e.g., Peaks A, B, C, D, fDOM, M, N, Z and T)
Trihalomethane Formation Potential
Haloacetic Acid Formation Potential
THMFP ug/L
THMFP ug/L
HAAFP ug/L
HAAFP ug/L
STHMFP Actual by Predicted
SHAAFP Actual by Predicted
Experimental Approach
Replicated Design: 3 treatments x 3 replicates = 9 total wetland cells
3 INFLOW water treatments
Fe – Iron Sulfate added prior to inflow
Al – Polyaluminum chloride added prior to inflow
Co - Untreated
THMFPs were measured in February, May, and July.
Like DOC, coagulation alone reduced THMFPs by %.
Passage through wetlands ADDED THMFPs for all cells (except Co Feb), especially Fe in May and July.
THMFP and DOC concentrations are highly correlated.
HAAFPs were measured in February, May, and July.
Like DOC, coagulation alone reduced HAAFPs by %.
Passage through wetlands ADDED HAAFPs for all cells, especially Fe in May and July.
HAAFP and DOC concentrations are highly correlated.
Two fluorescence EEMs from different source materials.
Other compositional indicators of DOM include the examination of ratios of different wavelengths (e.g., FI, HIX) and spectral slopes (S , S , S , S ) across specific regions of the optical spectrum, which can be related to DOM molecular weight, source, and processing
Best predictors for STHMFP, as identified by forward-stepwise regression analysis are SUVA254 SUVA280, SUVA350, SR, C:M, C:A, FI.
Best predictors for SHAAFP, as identified by forward-stepwise regression analysis are SUVA350 and β:α(-).
Water samples were collected at 2 locations:
INFLOWs (In) – immediately following coagulation treatment.
OUTFLOWs (Out) – water exiting each of the wetlands following a 3-6 day detention time.
Both THM and HAA precursors are very strongly linked to humic-like DOM which likely comes from peat soils.
Coagulation-Wetland Treatment System
Results - DOM Composition
Main Findings
Coagulation removes DOC and DBP precursors, especially HAA precursors.
Formation of HAAs and THMs are associated with high molecular weight (HMW), humic-like DOM.
Wetland passage adds DOC and DBP precursors, especially in summer Fe treatment.
Humic-like DOM is added during wetland passage after Al and Fe treatments. There is little to no compositional DOM change in the Controls.
DOM source during wetland passage likely comes from island peat soils rather than from new production (plant, algae).
Results suggest in situ coagulation is effective at improving drinking water quality, however some of the benefits gained from DOC and DBP removal can be negated during wetland passage, particularly in summer Fe treatments.
Discriminant Analysis (DA) was used to quantitatively determine which qualitative optical parameters could distinguish between the three treatments (Co, Al, Fe).
The nine parameters determined to be most significant (p<0.1) were SUVA254, SUVA280, SUVA350, , SR , SpB, C:M, C:A, FI, and β:α.
DOM
metal
Flocculate (floc)
Figure X: Location and design of experimental-wetland cells on Twitchell Island, California: A, aerial view of site design; B, wetlandcell
design (not to scale).
TWITCHELL ISLAND, CA, USA PEAT SOILS
Bird’s-eye view of the nine wetland cells Cell Dimensions 40 x 15 m
San Joaquin River
Principal Component Analysis (PCA) loadings were selected based on the nine significant qualitative optical parameters identified by Discriminant Analysis (DA).
Together PC1 and PC2 explained 88% of the total variance in the dataset.
PC1 (78.5%) showed strong positive loadings associated with fresh-like material (SpB, β:α, C:A, FI, SR) and strong negative loadings for humic-like material (SUVA254, SUVA280, SUVA350, C:M).
Coagulation removed humic-like DOM, leaving behind material characterized by higher FI and β:α.
Red arrows indicate a shift in DOM composition during wetland passage in Fe- and Al-treated wetlands in July to more humic-like DOM.
metal
Acknowledgements
Funding for this study is provided by the California Department of Water Resources and the USGS Cooperative Water Program. Special thanks to Robert Pedlar for making this study possible. We thank Nicole Stern, Yan Liang, Erica Schmidt, and Tad Doane for laboratory and field assistance.
Humic-like DOM Fresh-like DOM