Novel 'chemical cocktails' in inland waters are a consequence of the freshwater salinization syndrome

Abstract

Widespread changes in water temperatures, salinity, alkalinity and pH have been documented in inland waters in North America, which influence ion exchange, weathering rates, chemical solubility and contaminant toxicity. Increasing major ion concentrations from pollution, human-accelerated weathering and saltwater intrusion contribute to multiple ecological stressors such as changing ionic strength and pH and mobilization of chemical mixtures resulting in the freshwater salinization syndrome (FSS). Here, we explore novel combinations of elements, which are transported together as chemical mixtures containing salts, nutrients and metals as a consequence of FSS. First, we show that base cation concentrations have increased in regions primarily in North America and Europe over 100 years. Second, we show interactions between specific conductance, pH, nitrate and metals using data from greater than 20 streams located in different regions of the USA. Finally, salinization experiments and routine monitoring demonstrate mobilization of chemical mixtures of cations, metals and nutrients in 10 streams draining the Washington, DC-Baltimore, MD metropolitan regions. Freshwater salinization mobilizes diverse chemical mixtures influencing drinking water quality, infrastructure corrosion, freshwater CO₂ concentrations and biodiversity. Most regulations currently target individual contaminants, but FSS requires managing mobilization of multiple chemical mixtures and interacting ecological stressors as consequences of freshwater salinization.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Keywords: inland waters; non-point source water pollution; salinization; water quality.

Figure 1.

Trends in mean annual concentrations of base cations in rivers and lakes. Examples include rivers/lakes from USA, Canada, Europe, Russia and Iran. Citations of data sources are in the electronic supplementary material.

Figure 2.

High-frequency daily sensor data illustrating examples of significant relationships over months and years between (a) specific conductance and pH for rivers in the midwestern and eastern USA (Potomac, Mississippi, Kaskaskia, Big Muddy rivers are plotted on the left axis, while Missouri River and Difficult Run are plotted on the right axis), and (b) significant positive relationships between specific conductance and nitrate for some rivers, while other rivers had more complex patterns (electronic supplementary material, figures S2–S4). In Rock Creek, Maryland (c), there was a cyclical relationship between nitrate concentrations and specific conductance (measured every 15 min) over 14 days directly before, during and after a winter snow event.

Figure 3.

Increases in: (a) major and trace metals concentrations before, during, after snowmelt (spanning two months) in urban watersheds of the eastern USA demonstrated by routine monitoring of ambient concentrations in stream water, and (b) peaks in ambient Cu, Cd and Zn watershed exports (mass flux in stream per unit area of watershed) in Rock Creek in Washington, DC highlighted in yellow during winter months in response to road salting (examples from other nearby streams can be found in the electronic supplementary material, figure S6). (c) Experimental salinization of an urban stream with real road salt caused an experimental salt pulse to migrate quickly downstream; during the experiment, Na⁺ concentrations were elevated above baseline pre-disturbance conditions. (d) During the experiment, Cu concentrations peaked with Na⁺ concentrations as the experimental road salt pulse migrated downstream.

Figure 4.

Increases in concentrations of metals and nutrients in stream sediments exposed to varying levels of NaCl. There are negative dissolved metal concentrations reported in a few cases because we are comparing the values to incubations without salinization treatments as a control. Streams from 10 watersheds were located across the Baltimore, MD and Washington, DC metropolitan region [15] (electronic supplementary material, table S3). ISC denotes impervious surface cover in the watershed.