Title

Estimation of the Rate of Release of Base Cations via Chemical Weathering in Soils in the Catskills Region

Date of Award

August 2017

Degree Name

Master of Science (MS)

Department

Civil and Environmental Engineering

Advisor(s)

Chris E. Johnson

Subject Categories

Engineering

Abstract

Chemical weathering of minerals is the principal mechanism by which cations and other nutrients are supplied and acidity is neutralized in soils and drainage. Quantifying the rate of chemical weathering is therefore crucial for the calculation of “critical loads” of atmospheric acidity to forest ecosystems. The objective of this study was to estimate the rate of release of calcium (Ca2+), magnesium (Mg2+), potassium (K+), and sodium (Na+) through chemical weathering in soils in 25 headwater catchments in the Catskills region of New York. The release rates were estimated using two techniques: a watershed mass-balance approach and cation depletion in soil profiles using Zr as an immobile tracer.

Based on the mass-balance method, I estimated that Ca2+ was released at an average rate of 574 mol ha-1 yr-1 in the Catskills region (range 168 to 970 mol ha-1yr-1) and accounted for, on average, 60% of the total base cation weathering between 2010 and 2013. However, the cation depletion method indicated that Ca2+ has accumulated in Catskills soils at an average rate of 0.69 mol ha-1 yr-1 (range -0.49 to 3.01 mol ha-1 yr-1) in the period since deglaciation. These large differences suggest that modern-day weathering rates are much faster for Ca2+ and other base cations than long-term rates. The long-term trend of accumulation of Ca2+ indicates that external inputs, through precipitation and other atmospheric inputs, have exceeded weathering release in the period since the last glacial retreat.

Bedrock appears to influence both weathering rates and long-term depletion/accumulation of base cations; however, there is no clear relationship between the estimated modern weathering release rates and long-term mass fluxes in individual watersheds. The results of this study show that the modern-day weathering release rates remain elevated as a result of sulfate and nitrate deposition.

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