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Open-File Report 2009–1054

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### Prepared in cooperation with theState University of New York Research Foundation for theNew York City Department of Environmental Protection

## Nutrients, Dissolved Organic Carbon, Color, and Disinfection Byproducts in Base Flow and Stormflow in Streams of the Croton Watershed, Westchester and Putnam Counties, New York, 2000–02

#### By Paul M. Heisig

| ABSTRACT
The Croton Watershed is unique among New York City’s water-supply watersheds because it has the highest percentages of suburban development (52 percent) and wetland area (6 percent). As the City moves toward filtration of this water supply, there is a need to document water-quality contributions from both human and natural sources within the watershed that can inform watershed-management decisions.

Streamwater samples from 24 small (0.1 to 1.5 mi2) subbasins and three wastewater-treatment plants (2000–02) were used to document the seasonal concentrations, values, and formation potentials of selected nutrients, dissolved organic carbon (DOC), color, and disinfection byproducts (DBPs) during stormflow and base-flow conditions. The subbasins were categorized by three types of drainage efficiency and a range of land uses and housing densities.

Analyte concentrations in subbasin streams differed in response to the subbasin charateristics. Nutrient concentrations were lowest in undeveloped, forested subbasins that were well drained and increased with all types of development, which included residential, urban commercial/industrial, golf-course, and horse-farm land uses. These concentrations were further modified by subbasin drainage efficiency. DOC, in contrast, was highly dependent on drainage efficiency. Color intensity and DBP formation potentials were, in turn, associated with DOC and thus showed a similar response to drainage efficiency. Every constituent exhibited seasonal changes in concentration.

**Nutrients.** Total (unfiltered) phosphorus (TP), soluble reactive phosphorus (SRP), and nitrate were associated primarily with residential development, urban, golf-course, and horse-farm land uses. Base-flow and stormflow concentrations of the TP, SRP, and nitrate generally increased with increasing housing density. TP and SRP concentrations were nearly an order of magnitude higher in stormflow than in base flow, whereas nitrate concentrations showed little difference between these flow conditions. Organic nitrogen concentrations (calculated as the difference between concentrations of total dissolved N and of all other N species) was the dominant form of nitrogen in undeveloped and moderately to poorly drained subbasins.

High TP concentrations in stormflows (800–1,750 μg/L) were associated with well drained and moderately drained residential subbasins with high- and medium-density housing and with the moderately drained golf-course subbasin. Areas with medium to high housing densities favor TP transport because they provide extensive impervious surfaces, storm sewers, and local relief, which together can rapidly route stormwater to streams. SRP concentrations were highest in the same types of subbasins as TP, but also in sewered residential and horse-farm subbasins. The ratio of SRP to TP was typically a smaller in stormflow than in base flow. Base-flow TP and SRP concentrations were highest during the warm-weather months (May to October). The highest nitrate concentrations (3.0–4.5 mg/L) were associated with the urban subbasin and the three well drained, high-density residential subbasins. The two moderately drained lake subbasins and the two poorly drained (colored-water wetland) subbasins had consistently low nitrate concentrations despite low and medium housing densities. Nitrate concentrations were generally highest during the winter months and lowest during the autumn leaf-fall period. Organic N concentrations were highest during the leaf-fall period.

**Dissolved Organic Carbon.** DOC concentration was consistently highest in the two poorly drained (colored-water-wetland) subbasins and lowest in the well drained subbasins. Base-flow DOC concentration increased with decreasing drainage efficiency, except in the well drained sewered subbasin with high-density housing, where slightly elevated DOC concentrations throughout the year may indicate leakage from a nearby sewer main. Seasonal changes in stormflow DOC concentration were pronounced in all subbasins. An early November storm (after leaf fall) resulted in DOC concentrations that, in many subbasins, were two to three times greater than the next highest storm DOC concentration.

**Color.** Color (Pt-Co) was closely associated with elevated DOC concentration in the two poorly drained (colored-water-wetland) subbasins. These subbasins had base-flow medians 7 to 15 times higher than the base-flow medians in all of the well drained subbasins and half of the moderately drained subbasins. Color was also closely associated with stormflows: median stormflow intensities at most well drained and uncolored-water-wetland subbasins were about twice the base-flow intensities. Seasonal variations in color were evident in base flow and stormflow. Most residential subbasins with low median base-flow Pt-Co color showed late-fall peaks of nearly 30 Pt-Co color units (during and immediately after leaf off) and relatively stable Pt-Co color during the remainder of the year. Stormflow intensities followed the same pattern; the autumn color peaks were proportionally larger than the corresponding DOC peaks compared to other times of year, which suggests that DOC derived from autumn leaf litter is a better color source than at other times of year.

**Disinfection Byproducts.** Formation potentials of DBPs increased in base flow with decreasing drainage efficiency. Formation potentials of trihalomethane (THM) species and haloacetic acid (HAA) species, respectively, were added to determine total THMs and total HAAs. Chlorine-containing forms of THMs and HAAs produced in formation potential tests were much more common than bromine-containing forms. The bromine-containing forms, however, were associated with residential and golf-course land uses. THMs produced in formation potential tests were dominated by a single form (chloroform), whereas three forms of HAAs were common (monochloroacetic acid (MCAA), dichloroacetic acid (DCAA), and trichloroacetic acid (TCAA).

DBP formation potentials increased in base flow with decreasing drainage efficiency. THM and HAA formation potentials exhibited autumn (leaf fall) and late spring base-flow peaks; HAAs were higher in fall, and THMs were higher in spring. Stormflow THM and HAA formation potentials were typically high, irrespective of land use or drainage efficiency.

The major source of precursor material for DBPs in most subbasins is terrestrial DOC. Specific ultraviolet absorbance (SUVA) values in samples from subbasins with lakes indicate that the DOC is a mixture of both aquatic and terrestrial sources.

No universal surrogate for all DBPs (THMs and HAAs) was identified from DOC, UV-254, gelbstoff 440, or platinum-cobalt (Pt-Co) color. Samples were grouped by flow condition (stormflow or base flow) and by the presence or absence of recent leaf litter. The best surrogates for THMs differed from those for HAAs. The best surrogates for THMs were UV-254 and DOC, whereas the best surrogate for HAAs differed among the sample groups. THM-to-surrogate relations were generally stronger than HAA-to-surrogate relations probably because THMs have a single dominant form, whereas HAAs have three dominant forms which differed in concentration among subbasin categories.

**Wastewater Effluent.** The quality of wastewater effluent from three wastewater-treatment plants (tertiary treatment) differed with treatment process and plant upgrades. Nitrate and ammonium were the only constituents with concentrations that exceeded concentrations observed in streamflow samples. DBP formation potentials were low overall, but there were elevated concentrations of some brominated forms and of the HAA form monochloroacetic acid.

1 Citation of unpublished report: Heisig, P.M., 2008, Croton terrestrial processes project—final report, volume 1, chapter 4, broad brush study: New York City Department of Environmental Protection, 105 p., online only.      | -  **Report** PDF (5.9 MB) 
-  **Download Figure 9**pdf (2.7 MB)          Document size = 11" X 21.5"  

**For additional information contact:**Director,         New York Water Science Center          U.S. Geological Survey          425 Jordan Rd.           Troy, NY 12180 http://ny.usgs.gov/

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### Suggested citation:

Heisig, P.M., 2009, Nutrients, dissolved organic carbon, color, and disinfection byproducts in base flow and stormflow in streams of the Croton watershed, Westchester and Putnam Counties, New York, 2000–02: U.S. Geological Survey Open-File Report 2009–1054, 105 p., online only.

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### Content

 Abstract

 4.1 Introduction

 4.1.1 Subbasin Selection  

4.1.2 Sampling Frequency 

4.1.3 Representativeness of Stormflow Samples 

4.2 Concentrations of Selected Constituents Categorized by Drainage Efficiency and Land Use 

4.2.1 Total (Unfiltered) Phosphorus (TP) 

4.2.1.1 Summary: Median Concentrations in Base Flow, Stormflow, and Wastewater-Treatment Plant (WWTP) Effluent 

4.2.1.2 Base-Flow Concentrations, by Drainage Efficiency and Land-Use Category 

4.2.1.3 Stormflow Concentrations 

4.2.2 Soluble Reactive Phosphorus (SRP) 

4.2.2.1 Summary: Median Concentrations in Base Flow, Stormflow, and WWTP Effluent 

4.2.2.2 Base-Flow Concentrations, by Drainage Efficiency and Land-Use Category 

4.2.2.3 Stormflow Concentrations 

4.2.3 Nitrate 

4.2.3.1 Summary: Median Concentrations (as N) in Base Flow, Stormflow, and WWTP Effluent 

4.2.3.2 Base-Flow Concentrations, by Drainage Efficiency and Land-Use Category

 4.2.3.3 Stormflow Concentrations 

4.2.4 Dissolved Organic Carbon (DOC) 

4.2.4.1 Summary: Median Concentrations in Base Flow, Stormflow, and WWTP Effluent 

4.2.4.2 Base-Flow Concentrations, by Drainage Efficiency and Land-Use Category 

4.2.4.3 Stormflow Concentrations 

4.2.5 Color (Pt/Co) 

4.2.5.1 Summary: Median Intensities in Base Flow, Stormflow, and WWTP Effluent 

4.2.5.2 Base-Flow Intensities, by Drainage Efficiency and Land-Use Category 

4.2.5.3 Stormflow Color Intensities 

4.2.6 Unfiltered Trihalomethane (THM) Formation Potential in Filtered Samples 

4.2.6.1 Summary: Median Formation Potentials in Base Flow, Stormflow, and WWTP Effluent 

4.2.6.2 Base-Flow Unfiltered Trihalomethane Formation Potential, by Drainage Efficiency and Land-Use Category 

4.2.6.3 Stormflow Unfiltered Trihalomethane Formation Potentials 

4.2.7 Unfiltered Haloacetic Acid (HAA) Formation Potential 

4.2.7.1 Summary: Median Formation Potentials in Base Flow, Stormflow, and WWTP Effluent 

4.2.7.2 Base-Flow Formation Potentials, by Drainage Efficiency and Land-Use Category 

4.2.7.3 Stormflow Formation Potentials 

4.3 General and Species Specific Associations of Nutrients and Disinfection-Byproduct Formation Potentials with  Subbasin Drainage Efficiency, Land Use, Flow Conditions,and Season 

4.3.1 General Occurrence of Nutrients and Disinfection-Byproduct Formation Potentials 

4.3.2 Phosphorus and Nitrogen Occurrence and Relative Proportions of Species 

4.3.2.1 Total (Unfiltered) Phosphorus and Soluble Reactive Phosphorus

 4.3.2.2 Nitrate, Ammonium, and Dissloved Organic Nitrogen 

4.3.3 THM and HAA Species Produced by Formation-Potential Chlorination of Streamwater Samples—Identification and  Occurrence 

4.3.4 THM and HAA Occurrence with Respect to Flow Conditions, Subbasin Drainage Efficiency, and Land Use 

4.3.4.1 Base Flow 

4.3.4.2 Stormflow 

4.4 Sources of DOC and Potential Surrogate Analytes for Estimation of Disinfection- Byproduct Formation Potentials 

4.4.1 Characterization of DOC Sources 

4.4.2 Surrogates for DBPs 

4.5 Summary 

4.5.1 Total (Unfiltered) Phosphorus 

4.5.2 Soluble Reactive Phosphorus 

4.5.3 Nitrate 

4.5.4 Dissolved Organic Carbon 

4.5.5 Color (Pt-Co) 

4.5.6 Trihalomethane Formation Potential 

4.5.7 Haloacetic Acid Formation Potential 

4.5.8 DBP Sources and Surrogates 

4.5.9 WWTP Effluent 

4.6 Acknowledgments 

4.7 References 

4.8 Appendix 

### Figures 

1. Croton Watershed boundaries and hydrography, showing stream-network subbasins and sampling sites, southeastern  N.Y.

2. Base-flow total (unfiltered) phosphorus concentrations from July 2000 to July 2001, Croton Watershed, for all  drainage-efficiency, land-use, and housing-density categories. 

3. Base-flow soluble reactive phosphorus concentrations from July 2000 to July 2001, Croton Watershed, for all drainage-efficiency, land-use, and housing-density categories. 

4. Base-flow nitrate concentrations (as N) from July 2000 to July 2001, Croton Watershed, for all drainage-efficiency, land-use, and housing-density categories. 

5. Base-flow dissolved organic carbon concentrations from July 2000 to July 2001, Croton Watershed, for all drainage-efficiency, land-use, and housing- density categories.  

6. Base-flow Pt-Co color intensities from July 2000 to July 2001, Croton Watershed, for all drainage-efficiency, land-use, and housing-density categories. 

7. Base-flow unfiltered trihalomethane formation potentials from July 2000 to July 2001, Croton Watershed, for all drainage-efficiency, land-use, and housing-density categories. 

8. Base-flow unfiltered haloacetic acid formation potentials from July 2000 to July 2001, Croton Watershed, for all drainage-efficiency, land-use, and housing-density categories. 

9. Relation of nutrients and disinfection byproducts to land use and drainage efficiency at representative subbasins from July 2000 to July 2001, Croton Watershed, southeastern New York. Three-letter identifier on each plot indicates the representative subbbasin selected (table 1). 

10. Formation potentials of unfiltered (U) and filtered (F) trihalomethane (THM) species in (A) base-flow and (B) stormflow samples from the stream network, Croton Watershed, southeastern New York, 2000–02 

11. Formation potentials of unfiltered (U) and filtered (F) haloacetic acid (HAA) species in (A) base-flow, and (B) stormflow samples from the stream network, Croton Watershed, southeastern, New York, 2000–02 

12. Specific absorbance (SUVA) for January to mid-October, when recent leaf litter was absent from the study area, (A) base-flow and (B) stormflow samples from the stream network, Croton Watershed, southeastern, New York, 2000–02 

13. Best filtered THM formation potential surrogates (DOC and UV-254) for streamwater, differentiated by flow conditions (base flow, stormflow) and by the absence or presence of recent leaf  litter (Jan. to mid-Oct., mid-Oct. to Dec; respectively), stream network, Croton Watershed, southeastern New York, 2000–02 

14. Best filtered HAA formation portential surrogates (DOC, UV-254, and Pt-Co color) for streamwater, differentiated by flow conditions (base-flow, stormflow) and by the absence or presence of recent leaf litter (Jan. to mid-Oct., mid-Oct. to Dec, respectively), stream network, Croton Watershed, southeastern New York, 2000–02 

### Tables 

1. Drainage efficiency, land use, housing density, and sampling frequency for 24 subbasins and three wastewater-treatment plant sites in the Croton Watershed,southeastern, N.Y 

2. Median values of selected constituents in base flow and stormflow for each drainage efficiency and land-use category, and housing density in the stream network, Croton Watershed, southeastern New York, 2000–02 

3. Seasonal stormflow concentrations of total unfiltered phosphorus (TP) in the stream network by subbasin category, Croton Watershed, southeastern, N.Y., 2000–02 

4. Seasonal stormflow concentrations of soluble reactive phosphorus (SRP) in thestream network, by subbasin category, Croton Watershed, southeastern, N.Y., 2000–02 

5A. Seasonal stormflow concentrations of nitrate as N in the stream network by subbasin category, Croton Watershed, southeastern, N.Y., 2000–02 

5B. Comparison of maximum nitrate (as N) concentrations in stormflows among well drained subbasins with different housing densities for December and March storms, stream network, Croton Watershed, southeastern N. Y., 2000–02 

6. Seasonal stormflow concentrations of dissolved organic carbon (DOC) in the stream network by subbasin category, Croton Watershed, southeastern, N.Y., 2000–02 

7. Seasonal stormflow color intensity (Pt-Co) in the stream network by subbasin category, Croton Watershed, southeastern, N.Y., 2000–02 

8. Seasonal stormflow unfiltered trihalomethane (THM) formation potentials in the stream network by subbasin category, Croton Watershed, southeastern, N.Y., 2000-02 

9. Seasonal stormflow unfiltered haloacetic acid (HAA) formations potentials in the stream network by subbasin category, Croton Watershed, southeastern, N.Y., 2000-02 

10. Regression r2 values for filtered trihalomethane (THM) formation potential andfiltered haloacetic acid (HAA)  formation potential as functions of four surrogates in base-flow and stormflow samples from the stream network, Croton Watershed, southeastern, N.Y., 2000–02

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