Citation
Potential sources for the high nitrate levels in Stillhouse Hollow Spring

Material Information

Title:
Potential sources for the high nitrate levels in Stillhouse Hollow Spring
Creator:
Turner, Martha
Publisher:
City of Austin
Watershed Protection and Development Review Department
Publication Date:
Language:
English

Subjects

Subjects / Keywords:
Stillhouse Hollow Spring (Texas, United States) ( 31.0333, -97.5333 )
Environment ( local )
Resource Management ( local )
Genre:
Technical Report
serial ( sobekcm )
Coordinates:
31.0333 x -97.5333

Notes

General Note:
High nitrate levels were observed in Stillhouse Hollow Spring. These levels are of concern for aquatic life. Deformed salamanders were found in the spring run, and although the cause for the deformity is unknown, parameters with abnormal levels are being investigated and those levels are reduced where possible. To reduce nitrates, the source must be identified. Lawn fertilizer appears to be the most likely source of nitrates, while leaking sanitary sewer lines and bat guano are not major contributors.
Restriction:
Open Access
General Note:
See Extended description for more information.

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Source Institution:
University of South Florida Library
Holding Location:
University of South Florida
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All applicable rights reserved by the source institution and holding location.
Resource Identifier:
K26-03259 ( USFLDC DOI )
k26.3259 ( USFLDC Handle )
11503 ( karstportal - original NodeID )

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Karst Information Portal

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Full Text
Description
High nitrate levels
were observed in Stillhouse Hollow Spring. These levels are of
concern for aquatic life. Deformed salamanders were found in
the spring run, and although the cause for the deformity is
unknown, parameters with abnormal levels are being investigated
and those levels are reduced where possible. To reduce
nitrates, the source must be identified. Lawn fertilizer
appears to be the most likely source of nitrates, while leaking
sanitary sewer lines and bat guano are not major
contributors.



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Potential Sources for the High Nitr ate Levels in Stillhouse Hollow Spring By Martha Turner, P.E. Watershed Protection and Deve lopment Review Department Environmental Resource Management Division ABSTRACT High nitrate levels were observe d in Stillhouse Hollow Spring. These levels are of concern for aquatic life. Deformed salamanders were found in the spring run, and although the cause for the deformity is unknown, parameters with abnormal levels are be ing investigated and those levels are reduced where possible. To re duce nitrates, the source must be identified. Lawn fertilizer appears to be the most likely source of nitrates, while leaking sanitary sewer lines and bat guano are not major contributors. INTRODUCTION Stillhouse Hollow Spring has high nutrient concentrati ons. The average nitrate level is significantly above the levels in other springs and above the level of Aquatic Life Concern, 3.5 mg/L (see Table 1). It also exhibits peak nitrate levels above the dri nking water standard. Additionally, it has a higher proportion of deformed, sluggish, or dead Jollyville salamanders than in any other spring where they are found (COA 1999). Table 1. Average nutrients at some local sp rings and creeks, and standards and concerns Location Nitrate as N (mg/L) Orthophosphorus as P (mg/L) Stillhouse Hollow Spring 6.6 0.12 Barton Spring 1.5 0.02 Pit Spring (undeveloped) 0.1 0.01 Austin Area Creek Medians 0.3 0.05 Aquatic Life Concern 3.5 Drinking Water Standard 10.0 Fertilizer is the most likely source for the high nitrogen and phosphorus levels in Stillhouse Hollow Spring. Because of this, a community education program was provided to educate homeowners in the proper use of fertilizer. The goal was to cultivate healthy lawns with appropriate growth rates without allowing excess nutrients to enter either surface water or groundwater. However, the question of whether fertilizer actually is most responsible fo r the high nutrient level still remained. Several approaches were taken to identify the source of the high nutrients in the spring. Nitrate and orthophosphorus concentrations for all area springs were plotted to compare them with the relative severity and potential uniqueness of the high nutrients at Stillhouse Hollow Spring. Storm data was SR-05-06 Page 1 of 12 July 2005

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SR-05-06 Page 2 of 12 July 2005 examined to determine if the concentration patterns provided clues to sources. Tests were performed to conclude if bats, cave soil, leaking sewer lines, and/or fertilizer caused the problem. NUTRIENT CONCENTRATIONS IN AREA SPRINGS Nitrate levels in forty-two area springs ranged from th e detection limit up to 132 mg/L. Due of the broad range in the data, nitrates are plotted twice with differe nt scales for the nitrate levels (see Figures 1 and 2). Five springs that had nitrate levels greater than 10 mg/L are Roy Kiser, Driving Range, Blunn Preserve 1, Blunn Preserve 2, and Stillhouse Hollow. Twenty-four of the forty-two springs have mean baseflow nitrate concentrations greater than 3.5 mg/L. Six springs, Roy Kiser, Driving Range, Gentle, Elepha nt Ear, Cuernavaca, and Long Hog Hollow, have mean baseflow nitrate concentrations greater than those found at Stillhouse Hollow Spring. Three springs, Roy Kiser, Driving Range, and Blunn Preserve 2 have maximum baseflow nitrate concentrations greater than or equal to those found at Stillhouse Ho llow Spring. Thus it appears that Stillhouse Hollow Spring nitrates are high but not unique. The same can be said for the orthophosphorus concen trations. Due of the broad range in the data, orthophosphorus levels are plotted twice with differe nt scales (see Figures 3 and 4). Three springs that had orthophosphorus levels greater than 1.0 mg /L are Barton, Backdoor and Stillhouse Hollow. Fourteen of the forty-two springs have mean basefl ow orthophosphorus concentrations greater than or equal to 0.04 mg/L, the threshold used to indicate eutrophic conditions in lakes. Three springs, Backdoor, Bronc, and Log Hog Hollow, have mean baseflow con centrations greater than those found at Stillhouse Hollow Spring. However, all spring means are less than the TCEQ screening level of 0.5 mg/L. The maximum baseflow orthophosphorus c oncentration of 1.1 mg/L is found at Stillhouse Hollow Spring.

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Figure 1. Nitrate Concentrations in Area Springs SR-05-06 Page 3 of 12 July 2005

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Figure 2. Nitrate Concentrations in Area Springs with Truncated Nitrate Scale SR-05-06 Page 4 of 12 July 2005

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Figure 3. Orthophosphorus Concentrations in Area Springs SR-05-06 Page 5 of 12 July 2005

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Page 6 of 12 July 2005 SR-05-06 Figure 4. Orthophosphorus Concentrations in Area Springs with Truncated Orthophosphorus Scale

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STORM DATA Storm water samples were collected on June 7, 1996 during a 1.68-inch rainfall with 1.28 inches falling between 8 a.m. and 9 a.m. Based on conductivity leve ls, storm water flow consisted of approximately one-third aquifer discharge and two-thirds surface wate r. Turbidity and fecal coliform bacteria exhibited spikes in concentrations lasting one to two hours (see Figure 5). This may indicate that these parameters are most influenced by the first flush of overland runo ff, since runoff later in the storm is more dilute. Note that overland runoff enters recharge features and passes through conduits to the spring discharge point. Figure 5. Turbidity and Fecal Coliform Ba cteria at Stillhouse Hollow Spring following a storm. (Last data point is taken 5 days after the storm.) After the storm, conductivity and nitrate concentrati ons displayed an abrupt drop followed by a slow return to the pre-storm high levels (see Figure 6). The slow return to pre-storm nitrate levels may indicate that interflow though soil is leaching residual nitrate fro m fertilizer in addition to overland runoff. (It is suspected that most fertilizer leach ate enters the aquifer during lawn watering shortly after fertilizer application rather than during storms.) While this data does not provide direct evidence of a nutrient source, the patterns exclude rainwater and overland runo ff as the major sources of nitrate in the aquifer. SR-05-06 Page 7 of 12 July 2005

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Figure 6. Conductivity and nitr ate at Stillhouse Hollow spring following a storm. (Note samples taken 39 days before the storm and 5 days after the storm) BATS/CAVE LEACHATE Jackson et.al.(1999) examined twenty-one Edwards Plateau Caves including Cotterell Cave which is located near Stillhouse Hollow Spring (MAP) and th e Stillhouse neighborhood. No evidence was found of bats using the cave. Total nitrogen (TN) in Cotterell Cave soil was approximately 1.3% or 13,000 mg/Kg and surface soil samples in that area had similar levels. Cave soil in the other twenty caves all had lower TN levels, and those levels were lower than at the surface. Nitrate levels were not reported. City of Austin staff revisited the cave on 8/23/03, confirming the absence of bats, and sampling two types of cave soil: recent and paleolithic. The nitrates in cave soil averaged 23.7 mg/Kg (see Table 2) and the TN levels were less than those found by Jackson. However in the Stillhouse Fertilizer Leachate Study it was found that nitrate leachate is correlated to soil nitrate but not to total soil nitrogen. Table 2. Dry-weight soil concentrations from Cotterell Cave on August 23, 2003. Soil Type Nitrate+Nitrite mg/Kg Total Kjeldahl Nitrogen (mg/Kg) Total Nitrogen (mg/Kg) Total Organic Carbon (mg/Kg) Recent 21.2 3,930 3,951 62,300 Paleolithic 26.1 161 187 ND Cotterell Cave soil nitrate concentrations were comp ared to those in the Stillhouse neighborhood lawns. In the lawns, the average soil n itrate concentration ranges from 8.3 to 17.6 mg/Kg with 15% having nitrate concentrations greater than 20 mg/Kg. The st reet closest to the cave has soil nitrates ranging from 15 to 19 mg/Kg, which is similar to the 21.2 mg/K g level found recently in soil from Cotterell Cave. (Note that soil analysis methods are different for lawn soil and cave soil.) SR-05-06 Page 8 of 12 July 2005

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Nitrate leachate from recent cave soil is predicted to be 4.1 mg/L using a regression equation relating leachate and soil nitrates from unfertilized Stillhouse nei ghborhood yards in April (at the beginning of the growing season). (See SR-03-07 Stillhouse Fertilizer L eachate Study.) Based on the same equation, the average leachate from yards in January would range from 1.4 to 3.3 mg/L. Nitrate leachate from cave soil would be above the 3.5 mg/L level of concern, but not high enough to cause the 6.6 mg/L average concentration observed in Stillhouse Hollow Spring. Water flowing through the soil at the cave entrance is unlikely to also have passed through yard soil. LEAKING SANITARY SEWERS AND SEPTIC TANKS To assess the possibility that leak ing sanitary sewer lines are contributing to the high nutrients at Stillhouse Hollow Spring, caffeine and nitrogen isotopes were analyzed. Caffeine Caffeine is found in wastewater but not in any other local source. Normal levels of caffeine measured in domestic wastewater are between 20 and 300 g/L (Scott 2002). The caffeine concentration at Stillhouse Hollow Spring, on May 22, 2002, was below the detection limit of 0.01 g/L. A caffeine concentration of 0.01 g/L would indicate raw sewage diluted up to 2000 times. Normal levels of TN measured in untreated domestic wastewater are between 20 and 85 mg/L (Metcalf and Eddy, 1979). Raw sewage diluted only 13 tim es would produce the average Stillhouse Hollow Spring nitrate of 6.6 mg/l, assuming that all of the nitrogen converted to nitrate. Since caffeine was not detected at Stillhouse Hollow Spring, it appears that raw sewage is not a major source of nitrates in the spring. Nitrogen Isotopes The stable nitrogen isotope ratio 15 N/ 14 N ( 15 N) has been used to determ ine the source of nitrate in groundwater (Gormly and Spalding, 1979). Ranges of 15 N values for nitrate sources are shown in Table 3. Table 3. Ranges of 15 N values for nitrate sources Source Range Natural +2 to +9 per mil Fertilizer -2 to +7 per mil Animal Waste +10 to +23 per mil The 15 N value for Stillhouse Hollow Spring is 5.7, indica ting that animal waste is not a source. Pit Spring, in an undeveloped area of the Jollyville Plateau, has the same basi c geology and ecology as Stillhouse Hollow Spring and is therefore expected to have the same natural sources of nitrates. However, Pit Spring has a 15 N value of 7.6, which is consistent with natural sources and not fertilizer. The average nitrate concentration in Pit Spring is 0.1 mg/L, 66 times less than at Stillhouse Hollow Spring. The difference in the nitrogen isotope values and the difference in the concentrations at the two springs imply that fertilizer is the likely source at Stillhouse Hollow Spring. Atrazine in Fertilizer Atrazine is a persistent herbicide found in Weed and Feed fertilizers, which are typically about 1% atrazine by weight. For example, Scotts Lawn Pro Super bonus Weed and Feed is a 30-3-10 fertilizer containing 0.931% atrazine. If fertilizer is applied at a typical rate of 1 lb. N/1000 square feet, or 43.5 lbs/acre, atrazine is applied at a rate of 1.3 lbs/acre. It is estimated that 1% to 2% of the applied atrazine is lost to deep percolation (Lowery, B. DNR-66) SR-05-06 Page 9 of 12 July 2005

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The atrazine concentration at Stillhouse Hollow Spring, on May 22, 2002, was 0.025 g/L. Similar groundwater levels were observed in field studies of he rbicide practices. In a research study of cropping systems in Minnesota where atrazine was applied at 1.5 lbs/acre, the groundwater concentrations ranged from below detection to 0.07 g/L (Giebnik 2006). Atrazine concentrations in groundwater are influenced by many factors such as soil type, depth of soil and rainfall patterns. However, the presence of atrazine in urban baseflow spring discharge indicates that leachate from fertilizer is contaminating the aquifer. NITRATE SEASONAL PATTERNS The monthly baseflow nitrate concentrations for Still house Hollow Spring were investigated to determine if nitrate levels peaked during times when fertilizer a pplication is most likely (see Figure 7). Fertilizer is applied primarily in the spring, with a secondary appl ication in the fall. The seasonal nitrate pattern is somewhat similar to this, with peak s in the spring and an increase in mid-fall. Nitrate is at a minimum in July and November, relative to the surrounding mont hs, with local peaks occurring in February and September. Little growth occurs in early spring, a llowing more nitrate to reach the groundwater than in September, which is during the active growing season in Austin. However, both the seasonal flow pattern (see Figure 7) a nd the theoretical fertilizer pattern are similar to the seasonal nitrate pattern. A possible reason for this si milarity is due to the relationship between rainfall and leachate. Decreases in groundwater flow are associ ated with decreases in rainfall. With less rainfall there is probably less leachate and thus lower nitrate leve ls in the aquifer. But, regardless of whether the seasonal nitrate pattern is due to fertilizer application or rainfall, fertilizer is implicated. Figure 7. Baseflow Seasonal Patterns in Stillhouse Spring Nitrate FERTILIZER LEACHATE Leachate from fertilizer was collected from twenty-eight plots in nine yards in the Stillhouse neighborhood over five weeks during April through J une of 2004 (see SR-03-07, Stillhouse Fertilizer SR-05-06 Page 10 of 12 July 2005

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Leachate Study). Also, one set of samples was take n before fertilizing where the residual soil levels would primarily affect the leachate, and three sets were taken after fertilizing with the leachate affected by both the residual soil level and the fertilizer appli cation. The average nitrat e leachate was 4.7 mg/L. About one-third of the twenty-eight plots received no fertilizer, one-third received a natural organic fertilizer, 8-2-4, and one-third received the inorganic fertilizer, 21-0-0. Leachate was highest from plots fertilized with 21-0-0 and lowest from plots that were not fertilized. It is expected that more homeowners fertilize with inorganic fertilizers than with natural or ganic fertilizers. Thus the average leachate from the entire area is expected to be higher than the observed 4.7 mg/L. Fertilizer was applied at a low rate of lb./1000 sq. ft Since fertilizer is typically applied at rates of 1 lb./1000 sq. ft., typical leachate from yards is expected to be higher and could produce the 6.6 mg/L nitrate levels seen in Stillhouse Hollow Spring. SUMMARY Table 4 summarizes the evidence presented in the report. It suggests whether or not the data implies the source of the nutrients in the spring. Table 4. Does the data support the potential sources? Source Data Sewer Lines/Septic Bats/Cave Soil Other? Buried Waste Fertilizer Caffeine No Nitrogen Isotopes No Yes Cave Visit/Cave Soil No High Nitrogen in Other Area Springs Unlikely Maybe Atrazine Yes Seasonal Nitrate Pattern Maybe Storm Flow Maybe Fertilizer Studies Yes CONCLUSIONS There is no evidence to support that high nitrate levels in Stillhouse Hollo w Spring are from leaking sanitary sewer lines or bats, and cave soil alone coul d not produce enough nitrate leachate to cause the observed nitrate concentrations. Fertilizer is a potential source due to the presence of atrazine, the seasonal nitrate pattern, and the leachate nitrate concentrations measured during the Stillhouse lysimeter study. REFERENCES Gormly, J.R., and Spalding, R.J., 1979, Sources and concentrations of nitrate-nitrogen in ground water of the Central Platte region, Nebraska, Ground Water v. 17, p. 257-273. City of Austin, 1999, Jollyville Plateau Wate r quality and Salamander Assessment. In the Water Quality Report Series. COA-ERM/1999-01 City of Austin Watershed Protection and Development Review Department, Environmental Resources Mana gement Division. October 29, 1999. COA SR-03-07. Stillhouse Fertilizer Leachate Study by Martha Turner SR-05-06 Page 11 of 12 July 2005

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Lowery, B., McSweeney, K. Fermanich, K., Hart, G., Wang, D., Seybold, C. Effect of Soil Type, Selected Best Management Practices, and Tillage on Atrazine and Alachlor Movement through the Unsaturated Zone University of Wisconsin Water res ources Institute. Project Number DNR-66. Giebnik, Bruce. 2006. Management Systems and Ground Water Atrazine Concentrations University of Minnesota Extension service. Metcalf and Eddy. 1979. Wastewater Engineering: Treatment, Disposal, Reuse McGraw-Hill Boston. Scott, Troy M., Rose, Joan B., Jenkins, Tracie M., Farrah, Samuel R., Lukasik, Jerzy. Microbial Source Tracking: Current Methodology and Future directions Appl. Environ. Microlbiol. (2002) 68: 57965803. SR-05-06 Page 12 of 12 July 2005