The Impacts of Water Supply Processes on Macronutrient Cycling Across the United States
Creators: name: Flint, Elizabeth
Publisher: name: Lancaster University
Year: 2025
Subjects: Nutrient cycling
Water supply
Biogeochemical cycles

description: 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 NO<sub>3</sub>-N yr<sup>-1</sup>, 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<sup>-1</sup>, 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
 CO<sub>2</sub> supersaturated groundwater following withdrawal
 was estimated to emit 3.6 Tg CO<sub>2</sub>
 yr<sup>-1</sup>, an emission equivalent to 1.2% of
 CO<sub>2</sub> emissions from outgassing lakes and rivers.
 Degassing groundwater withdrawals are now estimated to be an important
 component of overall emissions of CO<sub>2</sub> on localised
 scales, with 45% of US counties estimated to have
 CO<sub>2</sub> 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
 CO<sub>2</sub> emissions have been detailed for the first time
 anywhere globally, with emissions due to irrigation groundwater withdrawal
 alone (2.6 Tg CO<sub>2</sub> yr<sup>-1</sup>)
 equivalent to 25% of the CO<sub>2</sub> emissions associated
 with electricity generation for irrigation groundwater pumping. The mass
 flux of phosphate (PO<sub>4</sub>) 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
 PO<sub>4</sub>-P yr<sup>-1</sup>. On a national
 level, subsequent leakage of water from watermains was estimated to
 release up to 2.6 kt PO<sub>4</sub>-P
 yr<sup>-1</sup> and 7.7 kt NO<sub>3</sub>-N
 yr<sup>-1</sup> 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 PO<sub>4</sub> dosed water
 from water supply networks due to outdoor water use at domestic residences
 was estimated to release up to 3.1 kt PO<sub>4</sub>-P
 yr<sup>-1</sup>, 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 &gt;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 PO<sub>4</sub>
 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 PO<sub>4</sub> 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 CO<sub>2</sub> 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.