Future Work

Limestone Consumption Model

In the environmental assessment documentation for this project, re-liming was anticipated. The following statement was made: "One threshold that may trigger consideration of re-liming is when the pH, acid neutralizing capacity (ANC), and calcium values in April under normal flow are down to 30% of the first year water quality improvement." The year 2003 was anything but a "normal" year, but the numbers were less than 30% of the first year treatment. In fact, assessment of the time for re-liming was an integral and continuous part of water chemistry data collection for the initial project period. As an example of the liming data evaluation, the calcium decay graph is provided (Figure 14). This graph was used to assess limestone consumption and predict the need for a second treatment as follows. Calcium increases in the

stream have been shown to be due solely to limestone dissolution. The graph is a plot of the difference between weekly calcium concentrations and the average background concentration versus the day number post liming. This graph fits an exponential decay plot, and gives the equation:

Y = 35.58 e^-(0.001146x)

The first 150 days of post treatment data were not used in generating this plot due to low flow conditions in the summer of 1999. After the tropical storms increased the flow in September of that year, the limestone treatment was activated. From the exponential decay plot, a consumption half-life was calculated to be 1.65 years. By using the first order kinetic decay model it was estimated that 93% of the added limestone was consumed by the second liming in November 2005. If the assumption is made that the kinetics are the same for the second liming as found for the first liming, then the 93% consumption level will be met in February 2012.

Figure 14: Calcium concentration by day number. Curve shows exponential decay for limestone consumption during the initial project period.

Recommendation for Third Liming

There were several reasons the mass of limestone introduced in the St. Mary's Wilderness from the first to the second liming was increased from 140 to 230 tons. The original estimate was made to provide for a five year treatment period and it was thought that the increase would extend the treatment period to seven years. In addition, more limestone was added to compensate for transportation losses, etc. Exponential curves for the two dosage amounts (Figure 15) reveal that diminishing returns occur after about three or four half-lives have passed (7 years). We estimate that about seven tons of limestone remained from the initial liming when the Wilderness streams were treated a second time. Using the decay model, we estimate that there will be about seven tons left from the second liming in 2012. If a third liming treatment is

done, we believe that any additional increase in mass of limestone over the 2005 amount will not significantly extend the treatment period to merit the increased costs, transportation, labor, Wilderness closure during liming or other associated issues.

Figure 15: Amount of limestone remaining (tons) by year since introduction. Curve shows exponential decay for limestone consumption.

The limited water quality data collected for St. Mary's since the 2005 project period do not provide a complete picture of how the second liming affected the stream chemistry. However, if it is assumed the model developed in the initial project period (and from other liming projects) is reliable, then the lack of extensive water chemistry does not handicap evaluation for a future liming project. In fact, the results of the several water quality samples that were collected support the model's prediction for the second liming, including the marked increase in pH, ANC, Ca/H, Ca/Mg and decrease in Al observed immediately after the limestone was introduced (see Results). The extensive water chemistry data collected throughout the St. Mary's Wilderness in April 2010 (Table V) provide the most useful information for assessing the acid base status of the streams in the Wilderness. The values for April 2004 and 2005 are included for comparison. Concentration values alone are indeterminate for assessing water quality and must be qualified by stream discharge. Unfortunately no discharge data were obtained in 2010 due to the absence of a flow gauge. However, discharge can be estimated by the amount of rainfall prior to the sample date (Table VI) since the yield is fairly constant in this watershed.

Table V: Water chemistry values for all sites sampled in 2010.

The discharge data collected at St. Mary's from 1999-2003 along with the historical rainfall data enabled the calculation of the discharge for April 19, 2004, April 16, 2005 and April 10, 2010. Pre-liming discharge values for the dates following February 4, 1999, March 9, 1999 and March 1, 1999, respectively, matched the post-liming dates. The first two post-liming dates were for five and six years, respectively, after the first liming. The other post-liming date was for five years after the second liming. Values for ANC, Ca/H and Ca/Mg were greater than pre-liming values for all these post-liming values, and Al was reduced, which was a goal of the liming project. Extrapolating to 2011 it may be expected that the limestone from the second liming will still be treating the stream, but with less improvement. Based on these values and the predicted limestone consumption described above, it is recommended that St. Mary's be treated with limestone a third time in the late winter of 2012 to maintain the water quality necessary for protection of aquatic life.

Table VI: Rainfall (cm) for time periods prior to selected April sampling dates five years post-liming. Rainfall recorded by precipitation station in Staunton, VA. (Data courtesy of wunderground.com).

Table VII: Water chemistry parameters found for SM 1 and discharge equivalent values pre-liming for five years (April 2004), six years (April 2005) and five years (April 2010) post-liming # 1 and # 2, respectively. All concentrations are in μeq/L, except pH (standard units) and Al_T (ppb).

Final Thoughts

One concern that was raised in the early years of stream liming is that aluminum would mobilize as the streams became more acidic when the limestone was consumed. It was said that this increase in Al_T would cause fish mortality. The data indicate that Al_T has not increased and remains well below the toxicity level (150 ppb). Another concern was that limestone would raise the pH of wetlands in the watershed that support Swamp Pink (Helonias bullata) which is a federally threatened plant that requires acidic conditions for survival. Limestone was introduced in the Wilderness in such a manner and in locations to avoid areas occupied by this plant. Water chemistry values (Table V, Swamp) collected from the discharge of the largest wetland where Swamp Pink is located showed no change.