Rock Creek Assessment

Chemical and Physical Indicators
The absence or presence of certain chemical properties will indicate whether Rock Creek and its tributaries are healthy or not

Temperature
is a very important for water quality and aquatic life.  Warm water holds less oxygen than cold water and can result in more plant growth and an increase in respiration rates.  Fish and aquatic insects are adapted to live within a range of temperatures.  Extreme changes in temperature can make aquatic life more susceptible to disease, parasites and pollutants. 

Water quality standards for temperature in a warm water fishery needs to be maintained between 4.4-30.5ºC (40-87ºF) depending on the time of year (PA Code Chapter 93). Warm water fisheries fare best at temperatures between 20-25ºC (68–77ºF).
 
How We Measured Temperature:
Water temperature was measured in-stream at the sampling site using field thermometers.

Our Results:
Rock Creek temperatures were generally very good and well within the range of a warm water fishery.  As would be expected temperatures were warmer in the summer months.  There were a few instances of values above 25ºC in July.  The state water quality criterion for that month is 30ºC and only the downstream Steven’s Run site (#3) exceeded this value.

Seasonal Pattern:
Temperature fluctuations appear to match the seasonal changes in air temperature. It is apparent that there was not a significant amount of warm water runoff during the test period.  Runoff from impervious surface can change water temperature.  

Temperature of the water regulates the quality of life in a stream as temperature of water is indirectly related to dissolved oxygen concentration


Adequate levels of Dissolved Oxygen are necessary for aquatic plants and organisms for respiration.

Cold water or trout fisheries should have readings of >5 mg/l; warm water streams like Rock Creek should have readings of >4 mg/l.

Oxygen enters water through diffusion from the air, tumbling water, and via production by aquatic plants during photosynthesis.  Bacteria that facilitate the decay process demand oxygen and an excess of plant growth and the resulting decay can lead to low oxygen levels (euthrophication). Levels are also impacted by the amount of light penetration and by stream flow.  Slow moving streams absorb less oxygen from the air and hence have lower oxygen concentrations.

Dissolved oxygen (D.O.) readings vary throughout the day and year. The lowest levels are just before dawn prior to the start of photosynthesis. Levels may also be affected by temperature. Cold water holds more oxygen than warm water. By computing
Percent Saturation
we compare the measured value with the maximum level of dissolved oxygen that can be in the water at a given temperature in the absence of other influences. 

A healthy stream should have percent saturation values between 80-120%.


Low values are usually the result of excessive bacterial decomposition of organic waste, which demands oxygen. Supersaturation (over 120%) is usually caused by too many nutrients promoting too much plant growth and photosynthesis or could be due to excessive aeration such as in areas downstream from large dams.
 

How we measured it:
We measured dissolved oxygen using a Winkler Titration. In this test the dissolved oxygen is “fixed” in the sample using reagents that form an acid compound. This compound is then titrated to a color change and the amount of titrant added is equivalent to the dissolved oxygen present in the sample.

Our results: Dissolved oxygen concentrations were generally good. Values for most sites never dipped below 5 mg/l.  Percent saturation values were more troubling with all sites having at least one reading of below 80%. These low values were generally during the summer months perhaps due to excessive plant growth and resulting decomposition.  The Stevens Run site (#2), which also had high nitrate concentrations, showed signs of supersaturation in March, and April.  Stevens Run (Site #3), and the sites below the sewage treatment plants (#4 and #5) also had some low values particularly during the summer months. Sites # 4 and #5 are located in pool habitats, which is probably a contributing factor to the low concentrations. The site below Lake Heritage (Site 6) also had some low percent saturation values in the summer months and generally had the lowest readings with a median value of 74%. This is probably due to the use of oxygen in the lake. Further studies on dissolved oxygen over the course of the day to determine daily variations may be warranted to determine the overall health of the Rock Creek.

Seasonal Patterns: Dissolved oxygen fluctuation roughly corresponds to the seasonal temperature changes.  Higher water temperatures in the late spring, summer, and early fall appear to influence D. O. as Oxygen remains dissolved in higher concentrations in the cooler months. 

Dissolved oxygen does not fall below the typical values associated with warm water fish classified streams.  Values of % D.O. saturation above 120% and below 80% may be a cause for alarm.  Supersaturation of oxygen in water may suggest excessive plant growth, and hyposaturation, (low concentrations), of oxygen in water may indicate too much bacterial decomposition or over nutrient enrichment. Median percent oxygen saturation values for all sites range from a low of 74% to a high of 102%.

Nitrate-Nitrogen is the form of nitrogen most commonly measured in aquatic studies.  Human sources include run-off from over-fertilized agricultural fields and lawns, sewage treatment plants, industrial discharges, and atmospheric deposition due to car exhaust and power plant emissions.

Nitrate–nitrogen (along with sulfate and orthophosphate) is an essential plant nutrient, but excessive levels can lead to too much algae and plant growth.  This excess growth can cause over-enrichment or eutrophication of the water.  Plants add valuable oxygen to water, but overgrowth may lead to a reduction in light and as plants die and decay there is an increase in bacteria that feed on the plants and use up the dissolved oxygen in the water.  As a result levels of dissolved oxygen may become low and impact aquatic life. 

Groundwater in the Gettysburg Lowlands has natural Nitrate concentrations of 1.1 – 3.7 mg/L.
 

In over-enriched streams nitrate-nitrogen may not be measurable in the water column because the nitrate can be concentrated in the algae growth.  Nitrate-nitrogen is controlled in drinking water sources because high levels (greater than 10 mg/l) can cause a condition in babies called infant methemoglobinemia (blue babies).  The blue appearance of the infant is due to interference with oxygen transport and can be fatal. Pregnant women and infants should not drink water with high levels of nitrates.

How we measured it:
Nitrate-nitrogen concentrations were measured using a colorimetric method in which nitrate is reduced to nitrite and the resulting color change is measured against a standard color.

Our results: Nitrate–nitrogen levels were very slightly elevated at most of the sites.  Stevens Run (Site 2) appears to be moderately impacted by urban runoff, with a median value of 1.5 mg/l.  The most downstream site on the mainstem  (#11) has the highest median concentration of 2.4 mg/l.  It may be impacted by the nearby golf course and agricultural lands.  The site below Lake Heritage (#6) has very low values - median of 0.2 mg/l -which indicates that plants in the lake may be utilizing most of the nitrate-nitrogen. 
 
Season Patterns:
There is a weak seasonal pattern with increased concentrations during the spring and winter, most likely due to rain and snowfall. Rock Creek’s nitrate levels stayed within a reasonable range during the seasonal changes in this year long study.  The seasonal fluctuations of nitrate levels varied only slightly from the 1.0 mg/l value typical of non-impacted streams.

Phosphorus does not easily dissolve in water, but may be bound to soil particles eroded off the land. Orthophosphate is the form of phosphorus most commonly measured in water quality studies. It is the form most readily available to plants and can be measured directly without digesting (acidifying and boiling) the sample.  Sources of orthophosphate include soil and rocks, sewage treatment plants, industrial discharges, over-fertilized agricultural fields and lawns.  Many detergents contain phosphates to enhance cleaning ability. In the 1990’s most states (including Pennsylvania) required the levels of phosphate to be reduced in most laundry detergents. The limitation on phosphates in laundry detergent does not include automatic dish detergent which still contains high amounts of phosphates.

Orthophosphate (along with nitrate and sulfate) is an essential plant nutrient. In fresh-water systems phosphorus is usually the nutrient in shortest supply in relation to plant needs.  As a result small increases in phosphorus can more easily trigger eutrophication than increases in nitrates.  However, nitrates are the limiting factor in estuarine systems, like the Chesapeake Bay, and hence it is important to limit nitrate discharges and runoff as well.

Groundwater in the Gettysburg Lowlands has natural orthophosphate concentrations of 0.005 - 0.15 mg/L.
  Natural Levels of greater than 0.1 mg/l may result in eutrophication.

There are not water quality standard criteria for orthophosphates, but Sewage Treatment Plant is reviewed on a case by case basis and limits of 1-2 mg/l are usually accepted.

How we measured it:
Orthophosphate concentrations were measured using a colorimetric method in which the resulting color change is measured against a standard color.

Our results: The orthophosphate levels measured in our study had median values of 0.06 – 0.43 mg/l.  All sites, except for upstream Stevens Run (Site # 3) had median values above 0.10mg/l – the level that indicates the potential for eutrophication.  The tributary sites had lower values, with an overall median of 0.18 mg/l, than the mainstem sites which had an overall median of 0.34 mg/l. The sites just below the Sewage Treatment Plants (#4 & #5) had some of the higher phosphate concentrations, with medians of 0.35 mg/l and 0.40 mg/l. Site 11 also had elevated concentrations with a median of 0.36 mg/l; this may be due to the surrounding land use (agriculture and golf course) and also the fact that this is the most downstream site.  The Lake Heritage site (#6) also had a relatively high concentration, with median value of 0.28 mg/l, which is an interesting contrast to the other low nutrient readings at this site.

Seasonal Patterns: Rock Creek’s orthophosphate levels exceeded the normal background level of 0.01 mg/l in 9 months of the year long study.  A close study of orthophosphates and transparency graphs may conclude that there does not appear to be a correlation between transparency and concentration of orthophosphates.  The slight increase in orthophosphates in the spring may be due to the high turbidity in March and April, and a similar interpretation may be made for the September and October high concentrations of orthophosphates.  The February peak in turbidity does not appear to cause a spike in orthophosphates in the following months.


Sulfates are found naturally in sedimentary rock and as a result of leaf decomposition (rotting). Human sources include rain and snow fall polluted by car exhaust and industrial pollutants; sewage treatment plants, tanneries, pulp, and textile mills. Runoff from over-fertilized agricultural lands may also be a source.  In coal mining areas acid mine drainage results in the creation of sulfates from metal sulfides.

Sulfate (along with nitrate and orthophosphate) is an essential plant nutrient and where concentrations are less than 0.5 mg/L plant growth will not occur. 

Groundwater in the Gettysburg Lowlands has natural sulfate concentrations of 2.7-44 mg/L. 

Sulfate is not usually toxic to animals or plants. Concentrations of below 250 mg/L are recommended for drinking water.

How we measured it:
Sulfate was measured by converting sulfate to barium sulfate crystals and measuring the resulting precipitate. 

Our results: In general, sulfate readings were slightly above what would be expected in natural waters, but not high enough to warrant concern. Sulfate values at the downstream Stevens Run site (#2) were generally the highest. This suggests that there may be impacts from stormwater or industrial discharges.  Values below the sewage treatment plants (#4 & #5) were somewhat elevated, an expected impact of sewage plant discharges.  Site #6, below Lake Heritage, had the lowest values.  Possibly, the plants growing in the lake are utilizing sulfate and as a result it is not present in the water at the spillway. 

Seasonal Patterns: There is an overall seasonal pattern with values in the summer being somewhat elevated and less variable.  Seasonal fluctuations of sulfate concentrations go well above the 5-50 mg/l indicated for naturally occurring levels of sulfate in aquatic systems. The jump in sulfate concentration above the natural limit occurred in early spring and remained high until the winter when sulfate levels returned to acceptable levels.  Late winter is the only time when sulfate levels were very low. 

Sources of sulfate may yield a clue as to why we see higher levels throughout the seasons.  Rock Creek’s substrate is largely bedrock, with many outcropping of shale that extends into the stream bed.  The sewage treatment plants, located on the stream, may well offer an infusion of sulfate with the treated sewage water released into the stream. Common agricultural fertilizers include ammonium sulfate, potassium sulfate, and sulfate of potash magnesia (potash) may contribute significant amounts of sulfate.


Transparency is a measurement of water clarity or turbidity.  Cloudy water can indicate runoff from construction, cultivated fields, pavement, industrial discharges or the presence of excess algae. Transparency measures suspended particles in the water that may reduce light penetration and hinder aquatic plant growth and dissolved oxygen production.  Streams have natural levels of transparency and fish and aquatic life are adapted to the varying levels present in the stream.  Problems arise when there is unusually cloudy (turbid) water for long periods of time. The longer the water remains highly turbid the more stressed aquatic life becomes.

There no statewide criteria for transparency
or turbidity, but coldwater fish such as trout are healthiest in streams with >20 cm of visibility or <40 NTU. Warm water fish are healthiest with >11 cm of visibility or <100 NTU.

How we measured it:
Water transparency was determined using vertical transparency tubes. In this test, water is collected in a 60 cm column and poured off until a disc at the bottom of the tube can be seen.  A value of 60 cm is clear water.  The lower the value the more turbid the water is.

Our results: Transparency values for Rock Creek ranged from 60 cm – 0 cm.  The median value for all sites was greater than 56cm and acceptable for Rock Creek’s warm water fishery.  Generally, the mainstem sites had a greater range of readings than those on the tributaries. This is to be expected due to the higher flow of the main stem. Since our sampling schedule was not based on rain events the transparency values may not be representative of the condition of Rock Creek after rainstorms.  A future study should focus on comparing transparency results at high and low flow.

Seasonal Patterns:
There is a weak seasonal pattern with more variability in March, April, August, and February. The low transparency values are likely related to rainfall and associated storm events.  Low transparency events are scattered throughout the year with a greater frequency in late winter and early spring. These lower values may be connected to spring runoff.