THE ROAD TO BROOK TROUT RECOVERY: CRAB-SHELL RESTORES ALKALINITY TO WATERSHEDS IMPACTED BY ACID DEPOSITION

Open Access
Author:
Caporuscio, Abby
Graduate Program:
Environmental Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 01, 2010
Committee Members:
  • Rachel Alice Brennan, Thesis Advisor
Keywords:
  • acid deposition; acid rain; crab shell; chitin; br
Abstract:
The effectiveness of crab shell versus traditional limestone rock for the passive treatment of acid deposition was tested in a series of batch microcosm, continuous-flow column, and field-scale road-side ditch and underdrain experiments. Contact with crab shell in batch microcosms quickly increased the pH of acid impacted stream water from 3.1 to 7.6, and increased the alkalinity from 0 to 37.2 mg/L as CaCO3 in as little as 6 hours. With increasing contact time, the pH and alkalinity continued to increase, reaching a maximum of 8.2 and 136.8 mg/L as CaCO3, respectively, after a 10 days of treatment. Under continuous-flow conditions, crab shell increased the pH of the water from 3.87 to 9.2 in just 2 days and then slowly returned to a circum-neutral pH of 7.5 for the remainder of the 54-day study. In comparison, columns containing limestone were able to increase the pH to a maximum of 8.55 by day 24, but then lost neutralizing capacity, ending at a pH of 6.01 after 54 days. Alkalinity followed the same trend as pH, increasing from 0 mg/L as CaCO3 to an average of 634 mg/L as CaCO3 in columns containing crab shell, yet only reaching a maximum of 22.8 mg/L as CaCO3 in columns containing limestone. Low levels of aluminum (0.6 mg/L) were easily removed from solution by crab shell, but broke through in columns containing limestone, eventually reaching influent concentrations within 55 days. Fermentation of crab shell released low levels of bioavailable ammonium (NH4+) into the water (< 18 mg/L as N), which may be helpful for restoring biological diversity in nutrient-deficient watersheds. Minimum loading criteria for crab shell and limestone to neutralize the acidic water examined in this laboratory study were determined to be 0.2 – 0.9 g/L and 13 – 60 g/L, respectively. Field-scale road-side ditches with crab shell in underdrains showed increases in pH, alkalinity, and aluminum removal of 1.19 units, 111 mg/L as CaCO3, and 0.466 mg/L, respectively, over a 3 month period. In comparison, treatment with limestone in ditches and in underdrains resulted in lower net changes in pH, alkalinity, and aluminum removal of 0.64, 6.13 mg/L as CaCO3, and 0.146 mg/L, respectively. This work suggests that crab shell may be an effective alternative substrate for the restoration of waters impacted by acid deposition due to its ability to provide excess alkalinity, remove metals, and provide trace nutrients, all with lower mass requirements and thus a smaller footprint than limestone.