Human-accelerated weathering increases salinization, major ions, and alkalinization in fresh water across land use

Abstract

Human-dominated land uses can increase transport of major ions in streams due to the combination of human-accelerated weathering and anthropogenic salts. Calcium, magnesium, sodium, alkalinity, and hardness significantly increased in the drinking water supply for Baltimore, Maryland over almost 50 years (p<0.05) coinciding with regional urbanization. Across a nearby land use gradient at the Baltimore Long-Term Ecological Research (LTER) site, there were significant increases in concentrations of dissolved inorganic carbon (DIC), Ca²⁺, Mg²⁺, Na⁺, and Si and pH with increasing impervious surfaces in 9 streams monitored bi-weekly over a 3-4 year period (p<0.05). Base cations in urban streams were up to 60 times greater than forest and agricultural streams, and elemental ratios suggested road salt and carbonate weathering from impervious surfaces as potential sources. Laboratory weathering experiments with concrete also indicated that impervious surfaces increased pH and DIC with potential to alkalinize urban waters. Ratios of Na⁺ and Cl^- suggested that there was enhanced ion exchange in the watersheds from road salts, which could mobilize other base cations from soils to streams. There were significant relationships between Ca²⁺, Mg²⁺, Na⁺, and K⁺ concentrations and Cl^-, SO₄^2-, NO₃^- and DIC across land use (p<0.05), which suggested tight coupling of geochemical cycles. Finally, concentrations of Na⁺, Ca²⁺, Mg²⁺, and pH significantly increased with distance downstream (p<0.05) along a stream network draining 170 km² of the Baltimore LTER site contributing to river alkalinization. Our results suggest that urbanization may dramatically increase major ions, ionic strength, and pH over decades from headwaters to coastal zones, which can impact integrity of aquatic life, infrastructure, drinking water, and coastal ocean alkalinization.

Figure 1

Geologic map of the Gwynns Falls watershed showing lithologic gradients and long-term sampling locations of the Baltimore LTER site.

Figure 2

Long-term trends in drinking water chemistry from Baltimore, Maryland, USA (data courtesy of Bill Stack and Baltimore Department of Public Works.

Figure 3

Concentrations of sodium in streams draining a land use gradient at the Baltimore LTER site over time.

Figure 4

Concentrations of major ions in streams draining a land use gradient at the Baltimore LTER site. The center vertical lines of the box and whisker plots indicate the median of the sample. The length of each whisker shows the range within which the central 50 % of the values fall. Box edges indicate the rst and third quartiles. Open circles represent outside values.

Figure 5

Examples of relationships between median concentrations of major ions and impervious surface cover in streams draining a land use gradient at the Baltimore LTER site over time. R² values for statistically significant relationships are: impervious surface cover vs. median concentrations of Na⁺ (0.86), Ca²⁺ (0.58), K⁺ (0.89), DIC (0.59), and pH (0.52).

Figure 6

Relationships between base cations and anions in streams draining a land use gradient at the Baltimore LTER site. R² values for all statistically significant relationships are: Na⁺ vs. Cl⁻ (0.97), DIC (0.08), SO₄²⁻ (0.17); Ca²⁺ vs. Cl⁻ (0.24), DIC (0.63), SO₄²⁻ (0.83); Mg²⁺ vs. Cl⁻ (0.17), DIC (0.52), SO₄²⁻ (0.78); K⁺ vs. Cl⁻ (0.54), DIC (0.03), and SO₄²⁻ (0.12).

Figure 7

Relationships between elemental ratios in streams draining a land use gradient at the Baltimore LTER site. Line represents 1:1 relationship for Na:Cl.

Figure 8

Examples of relationships between base cations and nitrate concentrations in some urban streams at the Baltimore LTER site. R² values for relationships at the DRKR site are: nitrate vs. Ca²⁺ (0.46), Mg²⁺ (0.57). R² values for relationships at the GFCP site are: nitrate vs. Ca²⁺ (0.68), Mg²⁺ (0.49). R² values for relationships at the GFGB site are: nitrate vs. Ca²⁺ (0.70), Mg²⁺ (0.80).

Figure 9

Changes in pH and dissolved inorganic carbon (DIC) in acidification experiments with concrete samples. DIC values at time zero are concentrations in ambient stream water at Pond Branch (a forest reference stream with no impervious surface cover) prior to any introduction of the concrete samples or experimental acidification. Solutions were then manipulated in the lab to produce different acidity levels: pH ~3 (upper panel), pH ~4.5 (middle panel) and pH ~7 (lower panel).

Figure 10

Longitudinal patterns in concentrations of Ca²⁺ and Mg²⁺ from individual grab samples significantly increased from the headwaters to the outflow of the Gwynns Falls at the Baltimore Long-Term Ecological Research (LTER) site (p<0.05). R² values for statistically significant relationships were: distance downstream vs. Ca²⁺ (0.56) and Mg²⁺ (0.16). DIC concentrations generally increased with distance downstream but were variable at a few headwater sites, and there were no significant relationships. However, there was a statistically significant relationship between DIC and distance downstream with the first 2 headwater sites removed (p<0.05), and the R² was 0.67. There was a significant linear increase in pH with distance downstream over the entire stream network (p<0.05) and the R² for distance downstream vs. pH was 0.56.