Long-Term Effects of a Watershed Liming Experiment on Soils and Surface Waters at Woods Lake, New York

In October 1989, approximately 1000 Mg of pelletized limestone was applied to two subcatchments of the Woods Lake Watershed located in the western Adirondacks of New York State. This experiment was designed to evaluate the effectiveness of calcite addition to watershed soils as a strategy to mitigate the effects of surface water acidification from acidic deposition. Woods Lake is 23 ha in size and lies within a 207 ha thin-till dominated watershed. In 1979 the lake was highly acidic (pH 4.8, ANC -8 μeq/L). The watershed treatment followed two earlier lake liming experiments. These experiments successfully raised the pH and ANC of the lake but only for a short time as the residence time of water in the lake is only 174 days. Because calcite applied to the soil would be continuously exposed to precipitation falling on the land surface, it was hypothesized that watershed liming would have a much longer impact. The limed subcatchments (102 ha) were treated with limestone that had been crushed to the consistency of fine sand and pelletized with a lignosulfonate binder to form pellets 1.41 to 4.00 mm in diameter. The chemical composition of the pellets was approximately 82% CaCO3, 8% MgCO3, 4% organic binder, and 6% inorganic salts and insoluble silicate minerals. Application rates were targeted at 10 Mg CaCO3/ha but limefall collectors measured lower rates with a mean of 7.85 Mg/ha falling in subcatchment II and 3.42 Mg/ha in subcatchment IV. During the first two years after watershed liming lake pH averaged 6.62, ANC 138 μeq/L and Ca2+ 119 μeq/L. Samples collected during the summer of 2005 show that, even after 16 years, the lake has maintained its positive ANC (37.7 μeq/L) and still has high concentrations of Ca2+ (151.7 μeq/L). A viable fish population has also survived in this previously fishless lake. Even more surprising, soil samples collected in 2005 still have significant quantities of undissolved CaCO3. In the time since treatment, the CaCO3 has been displaced downward through the organic horizon of the soil through seasonal deposition of organic debris from above. It now lies within an Oa horizon having a pH of approximately 4.5. Organic coatings on the CaCO3 are likely inhibiting reactions with soil water. Some reaction must still be occurring as the pH is higher and exchangeable acidity lower in treated catchments as compared to untreated control catchments. The pH of the organic horizons in the control catchments is as low as 3.6 while the exchangeable acidity is as high as 11.2 meq/100 gm. Exchangeable acidity of organic horizons in the treated catchments is generally less than 2 meq/100 gm. Calcite weathering rates are likely lower in this field experiment than expected due to the presence of organic coatings on the mineral grain surfaces that are formed during organic decomposition reactions in the soil. Despite this, the rate of Ca2+ release from the treated catchments is high enough to maintain a positive ANC in the lake.