The  Results

Hydronium Ion Concentration (pH) and Discharge

Values for pH are measured to find the hydronium ion concentration (acid) in water. Low values indicate acidic conditions, which can cause mortality due to stress in fish and other aquatic life. For a water body managed for brook trout, pH 5.5 is considered the minimum acceptable value, with values > 6 preferred. The limestone was added to raise the pH above these values from the low (pH < 5.5) values found upstream of the liming sites, and prior to liming for many downstream sites (Figure 2).

Figure 2: Map of St. Mary's Wilderness with pH and ANC values at sampled sites in January 1999 prior to the introduction of limestone.

Stream discharge is the volume rate of stream flow at a given point. The rate or velocity of stream flow is dictated by gradient and resistance of the streambed. The volume of stream flow is controlled by base flow, precipitation, vegetation, springs and tributaries. The two primary, variable affecters of discharge for the St. Mary's River are precipitation and vegetation. Summer months typically show low discharge due to the transpo-evaporation process used by vegetation for cooling, transporting groundwater into the atmosphere. Discharge is reported with pH because of the effect discharge has on pH following liming (Figure 3). At some point in time following a liming treatment, substantial discharge will cause the pH to rise dramatically due to the large amount of water dislodging the pile of introduced limestone sand and spreading it downstream. When the limestone sand is spread out it will dissolve more effectively due to the increased exposed surface area. Conversely, a low discharge shortly following a liming will not raise the pH very much because of the smaller surface area of the pile and reduced contact time. For most limestone treatment we have done, it has taken from a few days to several months for the introduced limestone to spread and incorporate into the stream substrate. Once that has occurred, the 150 – 250 m "treatment zone" will slowly be consumed as the stream flows through ie. when the treatment zone has established stream pH will be higher under low flow conditions and lower under high flow conditions due to velocity and contact time.

Figure 3: Values for pH (A) and discharge (B) for St. Mary's River SM 1. Discharge recording station was destroyed by Hurricane Isabel on Sept. 19, 2003 (Day 1644).

The pH and stream discharge were recorded at SM 1 weekly (Figure 3) during the primary phase of the project. There was a lengthy period during which no data were collected between the second liming, November 2005, and April 2010. The pH values were often less than the minimum acceptable value at the sampling site prior to the introduction of limestone (Figure 3A). The average value for the period prior to the first liming was pH 5.53 + 0.28. From the first liming until April 2005 the average pH was 6.08 + 0.32. This is a significant improvement that has benefited the aquatic life in the stream. There was substantial flow on the day the streams were limed, but the spring and summer of 1999 were extremely dry times with discharge significantly decreasing. In September 1999, several tropical depressions provided rainfall that increased discharge (Figure 3B). At this time the limestone bed that had mostly remained immobile during the summer months distributed in the streams and pH increased dramatically to values > pH 6. During the years that followed, pH generally decreased as the limestone was consumed. Storm events generally caused short-term decreases in pH, but episodic pH dips were not as low as they were prior to liming, thus the aquatic life was generally protected from hydronium ion stress.

Figure 4: Values for pH measured for eleven sites (SM 1 – SM 11) at St. Mary's River. Selected quarterly results.

Unfortunately the stream discharge data ended on September 19, 2003 when runoff from Hurricane Isabel destroyed the staff gauge. It was not replaced, but weekly samples for water quality were collected until April 2005. To supplement the water quality data obtained weekly at SM 1, ten additional sites along the St. Mary's River were sampled quarterly from January 1999 to April 2005. A graph of pH versus stream reach (Figure 4) for five selected quarters shows that both before and after liming, pH gradually increases upstream to downstream due to the increases in carbonate bearing minerals in the bedrock geology at lower elevations. The higher pH values were found in September 1999 for reasons explained above. April pH values for 2004 and 2005 were close to the pre-liming values. Only three water chemistry sample sites have been collected from St. Mary's since April 2005. Two samples were collected at SM 1 (before / after) the November 2005 liming, and the quarterly set taken in April 2010. The pH value increased dramatically immediately following the 2005 liming, but returned to near pre-liming values in the most recent results. These data suggest the second limestone treatment responded similar to the first treatment. April 2010 pH values throughout the watershed (Figure 5) are provided for comparison to the pre-liming values (Figure 2). These results indicate the St. Mary's River is ready to be limed again.

Figure 5: Map of St. Mary's Wilderness with the most recent (2010) pH and ANC values at sampled sites.

Acid Neutalizing Capacity (ANC) and Calcium/Hydronium Ratio (Ca/H)

Chemically available dissolved buffer (acid neutralizing capacity, ANC) in stream water samples is determined by acid titration and reported in equivalence of CaCO₃. The significance of the ANC is that it shows how close the stream is to becoming acidic and unsupportive of acid intolerant aquatic life. In fact, ANC is a better metric for assessing the acid base status of a natural water than pH as it is an absolute value (not logarithmic) and gives the amount of available buffer present. Another important metric, the calcium to hydronium ratio (Ca/H) value is calculated from the calcium concentration and the pH found for a water sample. Lower pH values can be tolerated by aquatic life when Ca/H ratio values are large. The Ca/H ratio is important because it indicates a level of protection for the gills of fish from aluminum absorption. The target value for St. Mary's liming project are > 25 μeq/L ANC and Ca/H ratio > 2, respectively.

Figure 6: Values for ANC (C) and Calcium hydronium ratio (D) for St. Mary's River SM 1.

Weekly SM 1 results for ANC (Figure 6C) and Ca/H (Figure 6D) show the same growth and decay for the project period as pH values. The ANC values were quite low for the St. Mary's River prior to liming, often showing negative values. The pre-liming ANC average was 2.1 ± 4.7 μeq/L. The low values are the result of a lack of carbonate bearing mineral in the Antietam Formation quartzite rock that makes up most of the St. Mary's Wilderness watershed. Thus little natural buffer is available to mitigate anthropogenic acidity from the atmosphere. The post-liming ANC values increased due to the slow dissolution of the introduced limestone sand to an average 24.4 ± 11.7 μeq/L. There was an ANC decrease during the 1999 drought coincident with the pH decrease described above, but an increase above target value of 25 μeq/L (marked on the graph) followed the 1999 drought. Except for some depressions in the years that followed caused by storm events, the ANC remained near 25 μeq/L until 2003 when an extended period of low values was observed due to the unusually high flows. Although the ANC values were less than the target value during this period it is likely that the values were not as low as would have been observed had mitigative liming not been done. The similar ANC values between the 1/99, 4/04, 4/05, and 4/10 (Figure 7), along with the ANC values for pre-liming (Figure 2) and in April 2010 (Figure 5), indicate that the St. Mary's is ready to be limed again.

Figure 7: ANC values measured for eleven sites (SM 1 – SM 11) at St. Mary's River. Selected quarterly results.

The Ca/H ratio was measured at 11 sites along the St. Mary's from the period prior to any treatment (1/99), a period shortly after the first liming (9/99), two periods at the end of the expected treatment lifespan (4/04 and 4/05), and the most recent sampling (4/10) (Figure 8). A generally accepted minimum value of 10 was chosen for this parameter (shown on the graph). Prior to liming, the Ca/H averaged 8.5 ± 4.8, while the average was 65.1 ± 52.0 post-liming. It is likely that Ca/H ratio would have been much less than 10 during the recent high flow periods without the mitigative liming. The similar Ca/H values between the 1/99, 4/04, 4/05, and 4/10 indicate that the St. Mary's is ready to be limed again.

Figure 8: Calcium to hydronium ratio measured for eleven sites (SM 1 – SM 11) at St. Mary's River. Selected quarterly results.

Calcium / Magnesium Ratio

Calcium and magnesium ions are naturally present in stream water from dust deposition or erosion of minerals in the watershed. Both ions are essential elements for fish survival and growth. Upstream of the liming sites and pre-liming data provide calcium to magnesium ration values that are reflective of the natural abundance of these two elements in the watershed. The limestone treatment was done with material that was almost entirely CaCO₃, so changes in calcium concentration not complimented by similar changes in magnesium concentrations indicate water chemistry changes directly resultant from the liming. The calcium to magnesium ratio values, rather than absolute concentrations, are reported to minimize the effects of low and high discharge events on the values.

Figure 9: Calcium to magnesium ratio at a treated site (SM 1, G) and an untreated control site (SM 11, H). Weekly results shown for SM 1; quarterly results shown for SM 11.

The effect of the first liming in 1999 is revealed by the marked increase calcium to magnesium ratio (Figure 9) immediately following liming. The limestone sand used for this study was high grade (>99% CaCO₃) and it contained essentially no magnesium, only calcium in the form of calcium carbonate. Like calcium, magnesium is a base cation and a group II metal that enters the stream water naturally from the weathering of the minerals in the soils and bedrock. Since the amount of magnesium would not have changed after liming, the calcium to magnesium ratio results from the increase in calcium concentration after liming due to limestone dissolution. The average calcium to magnesium ratio prior to liming was 0.75 + 0.04. Following the first liming the average ratio increased to 1.34 + 0.32 for the initial project period. The dramatic increase is due solely to limestone dissolution, not natural effects. The most recent Ca/Mg ratio values are similar to the pre-liming values (Figure 10) indicating that the treatment limestone has been depleted.

Figure 10: Calcium to magnesium ratio measured for eleven sites (SM 1 – SM 11) at St. Mary's River. Selected quarterly results.

Aluminum

Aluminum is a fish toxin as described above. A total aluminum concentration level above 130 ppb is considered hazardous for aquatic life and thus was chosen as the maximum acceptable amount for this study. The total aluminum concentration for the St. Mary's River was measured from the weekly samples taken at the gauging location (Figure 11E) and the quarterly aluminum values taken at the control site upstream of the limestone treatment (Figure 11F). The graphs show that aluminum was mobilized during high flow periods due to low pH and flushing in the untreated reach of the stream. Episodic short-term spikes in aluminum concentrations as well as the base flow concentrations were less than the target value downstream of limestone treatment. Aluminum concentration at site 1 averaged 39.3 ± 18.0 ppb prior to liming and 22.6 ± 19.7 ppb post-liming for the initial project period. Figure 12 shows aluminum concentration in relation to distance from the liming site.

Figure 11: Aluminum concentration values (ppb) at a treated site (SM 1, E) and an untreated control site (SM 11, F). Weekly results shown for SM 1; quarterly results shown for SM 11.

Figure 12: Aluminum concentration measured for eleven sites (SM 1 – SM 11) at St. Mary's River. Selected quarterly results.

Rainfall and Control Streams

Atmospheric acid deposition may have decreased over the ten year period of this project as shown by the weekly pH values of precipitation in the region (Figure 13). However this decrease has been too small to provide any significant increase in pH of the control streams, Spy Run and Cole's Run. These data support the conclusion that increases in pH (and other water quality parameters) in St. Mary's River and its tributaries during the initial project were due solely to the limestone introduction and unrelated to environmental changes.

Figure 13: Weekly pH values of precipitation, Spy Run and Cole's Run. Precipitation values taken from data collected at Charlottesville Station, VA00 (National Acid Deposition Program, 2010). A fitted trend line is included for precipitation pH.