remote sensing application in San Diego Luis De La Torre San Diego State University June 2012 June 2013 Dr. Trent Biggs San Diego State University

Transcription

1 Water quality and quantity during low flow in urbanized watersheds: A combined fieldwork and remote sensing application in San Diego Luis De La Torre San Diego State University June 2012 June 2013 Dr. Trent Biggs San Diego State University Submitted March 27,

2 Table of Contents Acknowledgements 3 Abstract 4 Objectives 5 Methods 6 Results 10 Conclusions 12 2

3 Acknowledgments This project was supported by Agriculture and Food Research Initiative Competitive Grant no from the USDA National Institute of Food and Agriculture. Dr. Trent Biggs for his continued support and guidance on this project and with me education. Lisa Thurn for her help with the chemical analysis and guidance in the lab. Dr. Chun-Ta Lai and Joshua Rambo for their help with the stable isotope analysis. 3

4 Abstract Urbanization is still a relatively new process that still has unknown consequences for our environment. While we have seen some effects of urbanization, such as increased peak flows and loss of storage due to impervious surfaces, certain effects like those on the nutrient cycle of a watershed are not well understood. Since the nutrient cycle affects not only the water quality, but also the vegetation and ecosystems adjacent to the stream, it is vital that we seek to understand what is going on with the water chemistry. However, as a result of the semi-arid desert climate of San Diego, there is another complication that arises from our high water demand. We currently import a large portion of our water supply from both the Colorado River and Northern California. It is unclear how much of this imported water is making it to natural water supplies, which makes understanding the stream chemistry difficult because it is unknown how much of the nutrients are coming from natural versus imported water sources. The purpose of this study is to help identify where the water in the San Diego River is coming from and to identify any correlations between land use, origin of water supplies, and nutrient levels in the stream. 4

5 Objectives The objectives of this study are: 1) to identify where the water in the San Diego River originates from, 2) to analyze nutrient levels in the San Diego River to better understand its stream chemistry and 3) to find any correlations between the water s origin, the stream chemistry, and land use. Identifying the origin of the water will be done using stable isotope analysis of Oxygen-18 (O18) isotope levels by using a natural water source as a reference to compare to the other water samples. Samples of water will be collected at various locations along the San Diego River s main stem and tributaries and then taken to the lab for analysis in order to get a comprehensive idea of where the nutrients in the stream are coming from. Data collected from both analyses will then be used to find correlations between how urbanized an area in the watershed is and how much of the water is natural versus imported and how nutrients concentrations correspond to those factors. 5

6 Methods Study Area In order to better understand the linkages between the different basins contributing to the main stem of the San Diego River, the sampling scheme was developed to capture variations in the land cover with regards to various urbanization, as well as to identify how much the tributaries contributed the main stem of the San Diego River. The sampling scheme (figure 1) includes eight points along the main stem of the San Diego River and two sampling points that are located towards the edge of two main tributaries, Forester Creek (red outline, Figure 1) and Alvarado Creek (purple outline, Figure 1). These tributaries were isolated because each has a land cover fraction of urbanized land higher than 80% (Figure 2; table 1) and we wanted to see 6

7 Figure 1. Map of the sampling scheme and drainage areas that pour into each sampling point. The blacked-out area in the NE corner of the map is the drainage area including the San Vicente and El Capitan reservoirs and everything up-stream. This area is hydrologically separated and, therefore, not considered in the analysis. if any relationships existed between the nutrient concentrations and the amount of urbanization in the watershed. The main stem of the river is primarily natural cobble and sand beds with extensive riparian vegetation growing along the banks, except in some small areas where the river passes under a major road. The tributaries, however, are almost entirely been channelized with only small sections of each tributary having parts of the channel left with natural cobble and sandy beds. For Forester Creek tributary, this was done intentionally to allow for the vegetation to provide ecosystem services in the form of natural habitat for wildlife and water filtration (Smith and Yagade 2010). 7

8 Figure 2. Map of the sampling scheme and drainage areas that pour into each sampling point overlay on top of generalized land use in the San Diego watershed. The blacked-out area in the NE corner of the map is the drainage area including the San Vicente and El Capitan reservoirs and everything up-stream. This area is hydrologically separated and, therefore, not considered in the analysis. Table 1. Watershed area statistics. Data presented in this table comes from GIS layers gathered from SANDAG and analyzed in ArcMap Site ID Site Name Area (km 2 ) Area Urbanized (km 2 ) Area Urbanized (%) 1 Carlton Hills Blvd % 2 Forester Creek % 3 West Hills Pkwy % 4 Mission Trails % 5 Mission Gorge % 6 SD Mission Road % 7 Alvarado Creek % 8 Ward Road % 9 Qualcomm Way % 10 Fashion Valley % Sample Collection 8

9 In order to understand how much each tributary was contributing to the main stem of the river, discharge was measured at each sample site on each collection day. Discharge was measured at each site by taking the product of the cross-sectional area and the flow velocity. Cross-sections were taken using a survey rod and the measurements were recorded and the area was calculated back at the lab. Flow velocity was measured using an orange peel and timing how long it took to travel five feet downstream. The distance was measured using a tape measure and the time was recorded on a stopwatch. The process was repeated three times and the average time was used in calculating discharge. Discharge was also gathered for two USGS-gaged locations from the USGS website in order to identify how accurate the calculated discharge was for each sampling. All samples were collected on a monthly basis for four months from October 2012 to January All samples for nutrient analysis were collected in 175-ml, polyethylene bottles with three samples being collected from each site. Samples were then labelled, and placed on ice until they could be taken home to be refrigerated before being taken to the laboratory for filtering and analysis. Samples were analyzed for Nitrate and Total Kjeldahl Nitrogen (TKN), in order to get the total Nitrogen, and for Total Phosphorus. All nutrient analyses were completed within 28 days of collection and were performed on a LaChat auto analyzer. One water sample was collected and filtered, in the field, into a 60-ml, polyethylene bottle from each site for O18 analysis. It was then labelled and placed on ice until it could be refrigerated and taken to the laboratory. The O18 samples were filtered two more times in the lab in order to remove any particulates that could damage the sampling needle on the analyzer. O18 analysis was done on a LGR liquid water isotope analyzer (Schade et al. 2005). 9

10 Results The discharge that we calculated over-predicts the actual discharge measured by the USGS on each sampling date (Tables 2 and 3). This is likely due to issues in measurement, possibly from both the cross-sectional area and the flow velocity. However, trends in the discharge are relatively well preserved. Table 2. Discharge derived from taking the flow velocity and the cross-sectional area. Sites and values in bold are gaged USGS locations (Table 3, below). Sampling Sites Discharge Site ID Site Name 10/24/ /27/ /30/2012 1/27/ Carlton Hills Blvd Forester Creek West Hills Pkwy Mission Trails Mission Gorge SD Mission Road Alvarado Creek Ward Road Qualcomm Way Fashion Valley Table 3. Discharge values taken from the USGS website from the two gaged sites in the San Diego River Watershed. These sites are also in bold in Table 2 (above). Sampling Sites Discharge Site ID Site Name 10/24/ /27/ /30/2012 1/27/ West Hills Pkwy Fashion Valley The samples for nitrogen show a definite trend with Forester Creek consistently having much higher total nitrogen concentrations than all the other sub-basins (Table 4), except for one 10

11 outlier in Alvarado Creek in November. The high nitrogen concentrations that day are unaccounted for and the concentrations return to more regular amounts after that sampling date. 11

12 Table 4. Total nitrogen and total phosphorus concentrations for each sampled sub-basin in the San Diego River Watershed. Sampling Sites Total Nitrogen Concentrations (mg/l) Total Phosphorus Concentrations (mg/l) Site ID Site Name 10/24/ /27/ /30/2012 1/27/ /24/ /27/ /30/2012 1/27/ Carlton Hills Blvd Forester Creek West Hills Pkwy Mission Trails Mission Gorge SD Mission Road Alvarado Creek Ward Road Qualcomm Way Fashion Valley

13 Also, it is important to note that the nitrogen concentrations in the watershed generally tend to decrease as it gets later into the wet season, particularly in the Forester Creek watershed between December and January, with the January sampling date coinciding with a storm. This trend indicates that the source of the nitrogen is likely a point source and not from urban runoff/nonpoint source pollution (Carpenter et al. 1998). There were no significant trends in the total phosphorus analysis. The O18 results were similarly inconclusive. The O18 concentrations showed some trends with both of the highly urbanized tributaries having some of the lowest O18 values in the first three months. However, there does not seem to be any correlations in the January sample set. Furthermore, the Mission Trails 2 reference sample site did not seem to be representative of a natural water source and so the endmembers could not be identified and the relative amounts of natural versus imported water could not be discerned from the samples. It is unclear if any conclusions can be drawn from the information gathered. Table 4. Delta 18O concentrations for each sampled sub-basin in the San Diego River Watershed. Mission Trails 2, Site 4.5, was the selected natural reference site for the 18O endmember analysis. Sampling Sites Processed Delta 18O (mean of 3 injections) Site ID Site Name 10/24/ /27/ /30/2012 1/27/ Carlton Hills Blvd Forester Creek West Hills Pkwy Mission Trails Mission Trails Mission Gorge SD Mission Road Alvarado Creek Ward Road Qualcomm Way Fashion Valley

14 Conclusions While it is difficult to discern much from the data, there are a few things that have become quite clear. First, the methods for measuring and calculating discharge need to be refined in order to produce more accurate and useful results. Ideally, we would be able to get a flow meter to record flow velocity in the stream, but the flow meter we had available was not able to measure the low flows in the upper parts of the watershed. Second, there is a clear pollution problem in Forester Creek likely coming from a point source in the watershed. Given that there is a large industrial area located just prior to the sampling location, it is possible that that may be the source of the pollution. It is not completely clear if the nitrogen concentrations are making their way to the main stem of the river since the mass balance could not be performed as a result of the inaccurate discharge values, but further research into the system and the riparian vegetation s effect on the sub-basin s stream chemistry could prove enlightening. Lastly, a more representative natural reference site for the O18 sampling needs to be identified in order to perform the analysis on the data. It could still be very useful to identify the origins of the river s water given that a majority of San Diego s water comes from imported sources and it is not completely clear how much of the water in the river is from urban runoff. 14

15 Bibliography Carpenter SR, Caraco NF, Correll DL, Howarth RW, Sharpley AN, and Smith VH Nonpoint pollution of surface waters with phosphorus and nitrogen. Ecological Applications. 8(3): Hibbs BJ, Hu W, and Ridgway R Origin of stream flows at the wildlands-urban interface, Santa Monica Mountains, California, U.S.A. Environmental & Engineering Geoscience. 18(1):51-64 Schade JD, Welter JR, Marti E, and Grimm NB Hydrologic exchange and N uptake by riparian vegetation in an arid-land stream. North American Benthological Society 24(1): Smith, J. S., & Yagade, G Transforming Forester Creek. Government Engineering, Retrieved from 15