The Impacts of Water Supply Processes on Macronutrient Cycling Across the United States

Flint, Elizabeth and Surridge, Ben and Gooddy, Daren and Ascott, Matthew J. and Stahl, Mason (2025) The Impacts of Water Supply Processes on Macronutrient Cycling Across the United States. PhD thesis, Lancaster University.

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Abstract

Anthropogenic activities modify biogeochemical cycles of the macronutrients carbon (C), nitrogen (N) and phosphorus (P) worldwide. This has resulted in a need for more effective nutrient management practices that are informed by accurate nutrient budgets. This thesis uses publicly available data to assess the impacts that human uses of fresh water across the contiguous United States (US) can have upon N, C and P cycles. Freshwater withdrawals were estimated to delay the downstream export of 417 kt NO3-N yr-1, a flux equivalent to 57% of total US river denitrification. Freshwater withdrawals were estimated to delay the downstream export of 15.1 Tg DIC yr-1, a flux equivalent to 51% of the total DIC exported to the oceans from the US by surface waters and subterranean groundwater discharge. On a national level, the degassing of CO2 supersaturated groundwater following withdrawal was estimated to emit 3.6 Tg CO2 yr-1, an emission equivalent to 1.2% of CO2 emissions from outgassing lakes and rivers. Degassing groundwater withdrawals are now estimated to be an important component of overall emissions of CO2 on localised scales, with 45% of US counties estimated to have CO2 emissions from degassing groundwater withdrawals that exceed those from major emitting facilities. Contributions of major water use sectors to freshwater withdrawal retention fluxes and degassing groundwater withdrawal CO2 emissions have been detailed for the first time anywhere globally, with emissions due to irrigation groundwater withdrawal alone (2.6 Tg CO2 yr-1) equivalent to 25% of the CO2 emissions associated with electricity generation for irrigation groundwater pumping. The mass flux of phosphate (PO4) added to potable water, for the purpose of minimising metal corrosion within US water distribution networks, was estimated as being up to 14.9 kt PO4-P yr-1. On a national level, subsequent leakage of water from watermains was estimated to release up to 2.6 kt PO4-P yr-1 and 7.7 kt NO3-N yr-1 into the environment, with these fluxes equivalent to 1.2% and 1.4 % of P and N from point sources, respectively. On a national level, the loss of PO4 dosed water from water supply networks due to outdoor water use at domestic residences was estimated to release up to 3.1 kt PO4-P yr-1, a flux equivalent to 1.4% of P input from point sources. Leakage and outdoor water use fluxes are a locally important sources of N and P across urban areas, with county-level leakage N fluxes >10% of leached agricultural N fertilizer inputs across 265 counties and combined leakage and outdoor water use P fluxes exceeding point source P inputs across 541 counties. This research suggests that N, C and P fluxes associated with water supply processes should be integrated into future local-level and sectoral nutrient budgets, policy making and management practices. For example, the loss of PO4 from water supply networks due to leakage has implications for economic level of leakage assessments, Lead and Copper Rule revisions and the sustainability of P use across the country. Similarly, the application of corrosion inhibitor derived PO4 due to outdoor water use may help inform state-level policy that seeks to regulate the use of P based lawn fertilizers at domestic residences. The contribution of degassing irrigation and public supply groundwater withdrawals to overall sectoral CO2 emissions may have implications for the sustainable use of groundwater, policy surrounding preferential irrigation method and the ability for water systems to achieve net-zero. The methodology developed here should be used to quantify similar fluxes across other countries with extensive freshwater use, as part of a global assessment.