Dissolved organic carbon and dissolved organic nitrogen concentrations and exports upstream and downstream of the Dallas–Fort Worth metropolis, Texas, USA

Abstract

Concentrations and export of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) from terrestrial landscapes to near-coastal zones vary with land use. Information on (DOC) and (DON) concentrations and exports from urban ecosystems is sparse; thus, their source from within urbanised watersheds such as soil or vegetation or from permitted sewage discharge is unknown. We examined DOC and DON concentrations and exports in four gauged subwatersheds in the humid subtropical, upper Trinity River basin, upstream and downstream of the Dallas–Fort Worth metropolis in Texas, USA. Annual average DOC concentrations ranged from 5.7±0.4 to 6.4±0.8mgL–1 and DON concentrations ranged from 0.31±0.05 to 0.33±0.14mgL–1. Dissolved organic carbon exports, which included permitted sewage discharge, ranged from 522kgkm–2 year–1 above Dallas–Fort Worth to 3637kgkm–2 year–1 below Dallas–Fort Worth. Permitted effluent discharge contributed between 1 and 35% of DOC loading above and below the Dallas–Fort Worth metropolis. DON exports ranged from 27 to 179kgkm–2 year–1 above and below Dallas–Fort Worth respectively. There was difficulty apportioning permitted effluent-discharge contribution to DON because of the transformations among nitrogen-species. A moderate but significant relationship was found between DOC and sodium concentrations (R2=0.45; P<0.0001; n=40) and between DOC and potassium concentrations (R2=0.45; P<0.0001; n=40). Dissolved organic nitrogen also displayed a significant relationship with sodium (R2=0.33; P<0.001; n=40) and potassium (R2=0.59; P<0.001; n=40), suggesting that increases in these cations to aquatic ecosystems may induce increases in DOC and DON concentrations. Although DOC export was significantly correlated with medium-density urban land use (r=0.96; P<0.05: n=4), DON export was not (r=0.93; P>0.05; n=4), suggesting that land-management practices and permitted point-source discharges have a significant effect on aquatic DOC and DON concentrations and exports derived from urban watersheds.

The importance of sewage effluent discharge in the export of dissolved organic carbon from U.K. rivers

“…Given the flux from STWs, it can be concluded that, at maximum, sewage effluent would comprise 3.6% of the DOC flux at the tidal limit of the United Kingdom. Aitkenhead‐Peterson and Steele () found that permitted effluent discharge contributed between 1% and 35% of DOC flux above and below the Dallas‐Fort Worth metropolis. Alternatively, Sousa, Salimon, Figueiredo, and Krusche () showed that the DOC in the Acre River of the Amazon Basin did not significantly change across urban areas, with the seasonality being the dominant driver of DOC changes.…”

“…Worrall, Burt, and Howden (2016) extended the assessment of fluvial carbon loss to consider loss of organic matter and the speciation of that loss as CO 2 or as CH 4 , and also the potential for the release of N 2 O to the atmosphere-both CH 4 and N 2 O are more powerful GHGs than CO 2 (Houghton et al, 1995). Several studies have shown that areas receiving urban drainage and especially sewage treatment discharge have higher GHG fluxes to the atmosphere than have those without urban influence and more natural land uses-from China (Wang et al, 2017) and from the United States (Aitkenhead-Peterson & Steele, 2016). Finlay, Johnson, and Worrall (2016) considered the role of drinking water abstraction from rivers and groundwater in diverting organic matter from rivers.…”

Nonpoint Source Contributions Drive Elevated Major Ion and Dissolved Inorganic Carbon Concentrations in Urban Watersheds

“…Water Na/Cl ratios of 0.58–0.71 in watersheds #2–#5, as compared to 1.10 in watershed #1 (Figure F), are evidence for Na + -driven exchange reactions. ,,,, These reactions are seen even in watershed #5, which has minimal stormwater detention structures. High Na + inputs are linked to breakdown of organic matter in urban soils and sediments with subsequent release of dissolved organic carbon and nitrogen. − Thus, road salt inputs may reduce the effectiveness of efforts to improve ecosystem functions such as denitrification through stream restoration or installation of stormwater detention and green infrastructure …”

High-Frequency Data Reveal Deicing Salts Drive Elevated Specific Conductance and Chloride along with Pervasive and Frequent Exceedances of the U.S. Environmental Protection Agency Aquatic Life Criteria for Chloride in Urban Streams

“…While in some cases, the declines correlate with increases in overall major ion concentrations (and SC), in other cases, the declines correlate with increases in a specific ion like chloride (Cl). ,,,− Elevated [Cl] can reduce water quality for drinking or irrigation purposes, e.g., by increasing the corrosivity of water, which, in turn, can cause increased concentrations of lead and other pollutants. , Inputs of water with elevated [Cl], and thus elevated SC, to lakes and ponds can cause reduction or cessation of seasonal mixing and alter biogeochemical cycles. ,, Deicing salts promote the breakdown of organic matter in soils and sediments, resulting in the release of nitrogen and phosphorus, − and increased fluxes of dissolved organic carbon and nitrogen are correlated with elevated sodium at the watershed scale …”