Relationships between land use and stream chemistry in the Mulberry River Basin, Arkansas

Authors: BURGESS-CONFORTI, JASON - Natural Resources Conservation Service (NRCS, USDA); Moore, Philip; Owens, Phillip; MILLER, DAVID - University Of Arkansas; Ashworth, Amanda; HAYES, PHILLIP - University Of Arkansas; EVANS-WHITE, MICHELLE - University Of Arkansas; ANDERSON, KELSEY - University Of Arkansas

Submitted to: River Research and Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2022
Publication Date: 5/11/2022
Citation: Burgess-Conforti, J.R., Moore Jr, P.A., Owens, P.R., Miller, D.M., Ashworth, A.J., Hayes, P.D., Evans-White, M.A., Anderson, K.R. 2022. Relationships between land use and stream chemistry in the Mulberry River Basin, Arkansas. River Research and Applications. 38(6):1031-1040. https://doi.org/10.1002/rra.3970.
DOI: https://doi.org/10.1002/rra.3970

Interpretive Summary: The Mulberry River, located in the Ozark National Forest in Northwest Arkansas, one of America's National Wild and Scenic Rivers, has been listed as impaired due to low pH since 2008. Stream chemistry may have been affected by the growth of conifers or by afforestation or conversion of native hardwood stands, which has been attributed to basin acidification in several regions and may be contributing acid to the Mulberry River basin. The objective of this study was to examine the relationship between land use (i.e., pine forest, hardwood forest, mixed forest, and pasture) and stream chemistry of 11 tributaries of the Mulberry River over a 2-year period. The average pH of the 11 tributaries increased with distance from the headwater sub-basin and ranged from 5.6 to 6.5. Coniferous (pine) forest land use was not correlated with stream pH nor was stream pH predicted by percent of forest in pine. Acid neutralizing capacity (ANC) and stream flow were negatively correlated with and somewhat related to percent forest in pine. The ANC was also correlated with total organic carbon which was negatively correlated with percent pine and positively correlated with percent hardwood. Deciduous land use was positively correlated with sulfate and negatively correlated with total nitrogen. Spearman's rank correlation and principal component analysis identified inverse relationships between stream pH and nitrate and between ANC and nitrate, which may suggest that nitric acid may be the primary source of acidity within the Mulberry River basin. Although no relationships were observed between percent pine and pH, conifer growth may be affecting the stream buffering capacity of the basin which would increase the susceptibility of the river to acidification.

Technical Abstract: The Mulberry River in Arkansas is one of America's National Wild and Scenic Rivers and has been listed as impaired due to low pH since 2008. Stream chemistry is directly related to land use and changes in land use can result in degradation of surface waters. Growth of conifers, through afforestation or conversion of native hardwood stands, has been attributed to basin acidification in several regions and may be contributing acid to the Mulberry River basin. The objective of this study was to examine the relationship between land use (i.e., coniferous forest, deciduous forest, mixed forest, and pasture) and stream chemistry of 11 tributaries of the Mulberry River over a 2-year period. Mean pH of the 11 tributaries increased with distance from the headwater sub-basin and ranged from 5.6 to 6.5. Coniferous forest land use was not correlated with stream pH (P > 0.05) neither was stream pH predicted (P > 0.05; R2 < 0.01) by coniferous forest land use. Acid neutralizing capacity (ANC) and stream discharge were negatively correlated (P < 0.05) with and partially predicted by coniferous land use (P < 0.05). Acid neutralizing capacity was also positively correlated with total organic carbon (r = 0.68) which was negatively correlated with coniferous land use (r = -0.27) and positively correlated with deciduous land use (r = 0.20). Deciduous land use was positively correlated with SO4 (r = 0.24) and negatively correlated with total N (r = -0.28) and NO3 (r = -0.31). Spearman's rank correlation and principal component analysis identified significant inverse relationships between stream pH and NO3 (r = -0.17) and between ANC and NO3 (r = -0.44), which may suggest that HNO3 may be the primary source of acidity within the Mulberry River basin. Although no relationships were observed between coniferous land use and pH, conifer growth may be affecting the stream buffering capacity of the basin which would increase the susceptibility of the river to acidification.