Altered Soil-Soil Water Interactions Inferred from Stream Water Chemistry at an Artificially Acidified Watershed at Bear Brook Watershed, Maine USA

Abstract

The Bear Brook Watershed in Maine, USA is the site of a paired watershed study. West Bear (WB) catchment is being artificially acidified with 1,800 eq ha⁻¹ y⁻¹ of (NH₄)₂SO₄. East Bear (EB) serves as the control. After six years of artificial acidification, volume-weighted concentrations in WB, normalized to EB, increased approximately as follows, in Reg L-¹: H⁺, 15; Al (umoles), 50; Al (p.eq L-¹), 100; Ca, 50; Mg, 20; Na, 10; K, 2; SO4, 120; NI-1,, 2; NO₃, 80; HCO₃ has decreased 10 paq L’. Based on changing chemistry, several inferences can be made about soil-soil water interactions.

• 1. Various combinations of cation pairs in stream waters from both catchments are significantly correlated on an annual basis. The strongest linear correlations (r² typically greater than 0.5), with positive slopes, occur for Mg versus Ca. These relationships suggest soil-soil water equilibria of the type: EquationSource % MathType!MTEF!2!1!+- % feaagCart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbWexLMBbXgBd9gzLbvyNv2CaeHbl7mZLdGeaGqiVu0Je9sqqr % pepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs % 0-yqaqpepae9pg0FirpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaai % aabeqaamaabaabauaakeaacaWGdbGaamyyamaaCaaaleqabaGaey4k % aSIaaGOmaaaakiabgUcaRiaad2eacaWGNbGaeyOeI0Iaamiwaiabg2 % da9iaad2eacaWGNbWaaWbaaSqabeaacqGHRaWkcaaIYaaaaOGaey4k % aSIaam4qaiaadggacqGHsislcaWGybGaai4oaiaadUeadaWgaaWcba % Gaam4qaiaadohacqGHsislcaWGnbGaam4zaaqabaGccqGH9aqpdaWc % gaqaamaabmaabaWaaSGbaeaadaWadaqaaiaad2eacaWGNbWaaWbaaS % qabeaacqGHRaWkcaaIYaaaaaGccaGLBbGaayzxaaaabaGaam4qaiaa % dggadaahaaWcbeqaaiabgUcaRiaaikdaaaaaaaGccaGLOaGaayzkaa % aabaWaaeWaaeaadaWcgaqaamaadmaabaGaamytaiaadEgacqGHsisl % caWGybaacaGLBbGaayzxaaaabaWaamWaaeaacaWGdbGaamyyaiabgk % HiTiaadIfaaiaawUfacaGLDbaaaaaacaGLOaGaayzkaaaaaaaa!6E76! ]]</EquationSource><EquationSource Format="TEX"><![CDATA[$$C{a^{ + 2}} + Mg - X = M{g^{ + 2}} + Ca - X;{K_{Cs - Mg}} = {{\left( {{{\left[ {M{g^{ + 2}}} \right]} \mathord{\left/{\vphantom {{\left[ {M{g^{ + 2}}} \right]} {C{a^{ + 2}}}}} \right.\kern-\nulldelimiterspace} {C{a^{ + 2}}}}} \right)} \mathord{\left/{\vphantom {{\left( {{{\left[ {M{g^{ + 2}}} \right]} \mathord{\left/{\vphantom {{\left[ {M{g^{ + 2}}} \right]} {C{a^{ + 2}}}}} \right.\kern-\nulldelimiterspace} {C{a^{ + 2}}}}} \right)} {\left( {{{\left[ {Mg - X} \right]} \mathord{\left/{\vphantom {{\left[ {Mg - X} \right]} {\left[ {Ca - X} \right]}}} \right.\kern-\nulldelimiterspace} {\left[ {Ca - X} \right]}}} \right)}}} \right.\kern-\nulldelimiterspace} {\left( {{{\left[ {Mg - X} \right]} \mathord{\left/{\vphantom {{\left[ {Mg - X} \right]} {\left[ {Ca - X} \right]}}} \right.\kern-\nulldelimiterspace} {\left[ {Ca - X} \right]}}} \right)}} $$ or, with assumptions: EquationSource % MathType!MTEF!2!1!+- % feaagCart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbWexLMBbXgBd9gzLbvyNv2CaeHbl7mZLdGeaGqiVu0Je9sqqr % pepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs % 0-yqaqpepae9pg0FirpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaai % aabeqaamaabaabauaakeaaceWGlbGbauaadaWgaaWcbaGaam4qaiaa % dggacqGHsislcaWGnbGaam4zaaqabaGccqGH9aqpdaWcgaqaamaadm % aabaGaamytaiaadEgadaahaaWcbeqaaiabgUcaRiaaikdaaaaakiaa % wUfacaGLDbaaaeaadaWadaqaaiaadoeacaWGHbWaaWbaaSqabeaacq % GHRaWkcaaIYaaaaaGccaGLBbGaayzxaaaaaaaa!50A1! ]]</EquationSource><EquationSource Format="TEX"><![CDATA[ $${K-_{Ca - Mg}} = {{\left[ {M{g^{ + 2}}} \right]} \mathord{\left/{\vphantom {{\left[ {M{g^{ + 2}}} \right]} {\left[ {C{a^{ + 2}}} \right]}}} \right.\kern-\nulldelimiterspace} {\left[ {C{a^{ + 2}}} \right]}}$$ The value of K’_Ca-Mg remains relatively constant through time in both watersheds, except in WB in and after the fourth year of the manipulation of WB. Thereafter there is preferential depletion (Mg>Ca>Na>K), primarily along shallow flow paths — thus altering the solid activity ratios of the exchange surfaces. In EB, base cation concentrations decline with increasing discharge (increasing H⁺), due to dilution and interaction with soils with lower base saturation. In WB the acidification reverses this relationship, perhaps partly because of displacement of cations by NH₄, from the amendments. With progressive depletion of Ca and Mg in the quick-flow paths, concentrations start to decline at higher discharge, in spite of lower pH.

• 2. Sulfate concentrations increased in WB to as high as 230 µeq L⁻¹ at high flow. The percentage of added SO₄ leached to the stream increased to approximately 65% by the end of 1995. Thus, soils along base-flow paths adsorbed about 35% of the added SO₄ in 1995.

• 3. Aluminum concentrations in WB have increased from a pre-manipulation maximum of 10 µmole/L at high flow to 60 µmole/L. The relationship between Al and H⁺ is: EquationSource[ % MathType!MTEF!2!1!+- % feaagCart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbWexLMBbXgBd9gzLbvyNv2CaeHbl7mZLdGeaGqiVu0Je9sqqr % pepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs % 0-yqaqpepae9pg0FirpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaai % aabeqaamaabaabauaakeaacaWGbbGaamiBaiabg2da9iaaicdacaGG % UaGaaGymaiaaiodadaqadaqaaiaadIeadaahaaWcbeqaaiabgUcaRa % aaaOGaayjkaiaawMcaamaaCaaaleqabaGaaGOmaaaakiabgUcaRiaa % isdacaGGUaGaaG4maiaaiwdaaaa!4CF1! ]]</EquationSource><EquationSource Format="TEX"><![CDATA[ $$Al = 0.13{\left( {{H^ + }} \right)^2} + 4.35 $$ which could result from either desorption or dissolution of Al to a 2+ specie. This relationship has been relatively constant through the manipulation. The Al/Ca molar ratio increased from pre-manipulation values of 0.1 to 0.3 to 0.8, at higher flow.

• 4. The minimum pH in WB, achieved at highest flow, has decreased from about 5.3 to <4.7, an increase of about 15 µeq H⁺L⁻¹. The increase in H⁺ has been approximately 2 µeq L⁻¹ yr⁻¹. Neutralization of acidity has been initially accommodated by mobilization of Ca>Mg>AI>Na>K>H; by 1995 the neutralization involves the release of Al>Ca>Mg>Na≅H>K. Thus, the soils are inferred to (1) have reduced base saturation, (2) preferential proportional loss of Mg over Ca, (3) increased SO₄ saturation, and (4) higher exchangeable acidity.