Summary
The purpose of this project was to determine the source(s) of high nutrient concentrations, which had previously been observed and documented in Goleta Slough. Data collected during this study indicates that the predominant source of the high nutrients is runoff from agricultural areas in the Slough’s drainage basin, and that the agricultural area that contributes the majority of the excess nutrients is that adjacent to Tecolotito Creek, in Glen Annie Canyon above Cathedral Oaks Boulevard. The data further suggests that the nutrient source in Glen Annie Canyon is not readily depleted, but remains strong even during heavy storm runoff, whereas that along Carneros Creek can be depleted by heavy storm runoff, but is subsequently regenerated.

Sampling Locations and Dates
Samples were collected at various locations along the two major creeks that provide drainage into Goleta Slough (see Figure 1), Tecolotito Creek (“T” sampling stations) and Carneros Creek (“C” sampling stations), as well as near the Slough outlet (station S1), on 8 separate days over the months of March, April, and May, 2000. (Photographs of the sampling locations may be viewed.) The nutrient concentrations measured in these samples are shown in Tables 1 and 2. Samples taken on March 20 and 23, April 12, and May 17, were associated with moderate-to-low flow conditions typical of late spring in the Goleta Slough watershed, while those taken on April 17, 18, 19, and 21, were associated with a major storm event with heavy-to-moderate runoff.

Although the concentrations of all nutrients (nitrate, phosphate, ammonium) are elevated in Goleta slough and its tributaries, relative to natural levels (see the discussion in the Project Proposal), the degree of elevation is much more pronounced for nitrate. For this reason, the following discussion will focus mainly on nitrate levels.

Early Sampling – Dry-Weather Conditions
The first sampling episode, March 20, was used to survey the watershed, to determine the relative nutrient inputs from the two major drainages, as well as which areas within each of the drainages might be providing more of the input than other areas. Although both creeks were seen to have high levels of nutrients, the results (Table 1) clearly showed that Tecolotito Creek was by far the stronger source, with over 4 times higher concentrations of nitrate (1.46 millimolar (mM)) than Carneros Creek (0.32 mM) near their points of entrance into the Slough (stations T1 and C1, respectively). (Exact flow rates in the two creeks were unfortunately not measured, but the volume of water entering the Slough from Tecolotito Creek was visibly greater than that from Carneros Creek.) It was also clear that the vast majority of the nutrients entered Tecolotito Creek within the heavily agricultural region along Glenn Annie Road above Cathedral Oaks Boulevard: nitrate concentration increased from 0.04 to1.56 mM between stations T4 and T3, while changing very little between T3 and T2, and decreasing slightly between T2 and T1.

On March 23, additional samples were obtained along Tecolotito Creek. Samples taken from the two branches of the creek above station T4 indicated that the water at T4 (nitrate concentration .03 mM) is a roughly equal mixture of water from the left-hand branch (T4.1, .05 mM), and water from the right-hand branch (Tc, .01M). The nitrate concentration at station T3 on this date (1.84 mM) was somewhat higher than that on March 20.

About 3 weeks later, on April 12, another set of samples was taken along Tecolotito Creek. Nitrate concentrations had increased significantly at both T4 (0.15 mM) and T3 (2.38 mM). The increases in concentration were probably due to the fact that stream flow decreased over this time period, while the sources of nitrate input remained relatively constant. In order to narrow the range of possible input sources to Tecolotito Creek, an additional sampling station, T3.1, where the road to Glen Annie Golf Course crosses the creek, was introduced at this time. The nitrate concentration at this station (2.76 mM) was greater than that at station T3, indicating that the source(s) of high nitrate was still upstream from this location. Unfortunately, there were no other public-accessible locations along Tecolotito Creek between stations T4 and T3.1, since this part of the creek is on private land. Therefore, a more precise determination of the source(s) of high nitrate input to this creek was not possible at this time. Additional sampling locations upstream from station C3 on Carneros Creek were also precluded due to the lack of public access to this area.

Rainstorm Event and Related Sampling
In the early morning of April 17, a strong rainstorm passed through Santa Barbara County, resulting in very strong runoff in the two creeks involved in this study (as well as elsewhere). The peak rainfall from this storm occurred around 7:00 am. Samples were taken at our Tecolotito and Carneros stations between 9:00 and 10:00 that morning, and a series of hourly samples were taken at station S1, at the mouth of the Slough, starting at 9:00 am. (All stated sampling times are +/- 15 minutes.) The two creeks were also sampled on the following two days (April 18 and 19), and hourly samples were again taken at station S1. A single sample was taken on April 19 from the drainage adjacent to Mesa Road, station M1. Two days later (April 21), a limited sample set was obtained from Tecolotito Creek, and a single sample was taken at station S1. The results from these sampling efforts provide some insight into the nature of the high nutrient input sources.

Three general effects are discernable from this data (Tables 1 and 2). First, the nitrate concentrations in both creeks were lowered immediately after the rain event. Second, the various sampling stations showed one of two opposite trends in nitrate concentration as the runoff from the storm subsided, either increasing or decreasing. And third, the concentrations at the mouth of the Slough (S1) were essentially constant each day, except for the first sample (9:00 am) on the first day (April 17).

Regarding the first observation, it is not surprising that the nutrient concentrations were lower following the strong rain event, since the volume of water flowing down the creeks was at least 100 times greater than that prior to the storm (visual estimate), and a dilution effect would be expected. What is surprising is that the concentrations did not drop more strongly: values on the morning of April 17 fell by factors of only about 3 to 8 from the values measured prior to the storm, rather than by factors around 100. This clearly shows that the additional storm runoff that was feeding the creeks was bringing with it increased quantities of nutrients.

The second observation points to two types of nutrient source, each with different characteristics. In the case of the Tecolotito Creek source, where concentrations rose as the storm runoff subsided, the source behaves as if it is inexhaustible. There is either an extremely large reservoir of leach-able nitrate in this region, or else the leached nutrients were immediately replaced by re-application of fertilizers; the concentration increase observed over time would most likely be due to the decreasing volume of water as the runoff subsided. In the case of Carneros Creek, where the concentrations decreased over the two days following the storm, the nutrient source was clearly being depleted by the storm runoff. The return to higher levels a month later, however, indicates that there was a replenishing of the source during this period, probably via a re-application of fertilizers.

The third observation verifies the strong storm runoff and its minimization of tidal effects at the Slough outlet location (S1). Most of the year the water parameters at this location, including nutrient concentrations, are heavily influenced by the tides. The values obtained on May 17, plotted in Figure 2, show the more typical variations. The one data point that does not follow either of these expected patterns is that obtained at 9:00 on the morning of the storm: the nitrate concentration at this time was significantly higher than that of subsequent samples. We attribute this to the 9:00 am sample being at or near the crest of the storm runoff, with the accumulated nitrate residues from the watershed reaching a maximum concentration. Even though the concentrations in Tecolotito Creek rose during the next two days, the concentrations at S1 continued to drop, suggesting that the majority of the widespread runoff from this major storm event had decreasing levels of nitrate, as seen in Carneros Creek.

Continuous Sampling – In Situ Nitrate Analyzer
At the start of this project, it was hoped that a semi-continuous recording in situ nitrate analyzer, deployed at the Slough outlet (S1), would be of major help in correlating the changes in nitrate concentrations occurring in the watershed with those in the Slough. A major financial commitment of this project was the one-month rental of such an in situ analyzer. However, due to a series of unfortunate delays both on campus and at the manufacturer’s facility in England, the instrument was not received here until early May, and the first opportunity to deploy the unit, after familiarization with its operation, was not until May 12. Retrieval of the first three days of data (noon, May 12, through about noon, May 15), with the machine having taken readings once every 15 minutes, showed reasonable-looking data with the expected tidal influence (Figure 3). Re-deployment of the unit and subsequent retrieval of the data from May 15 – May 18, however, was very discouraging. A combination of low battery levels and mechanical problems caused the periodic internal calibration of the instrument to be very unstable, resulting in essentially meaningless values for the nitrate concentrations in the Slough. At this point, prior commitments by the researchers on this project, along with the fact that the rental time for the unit was running out, precluded the possibility of remedying the problems and re-deploying the instrument. Thus, the in situ data was of little help in evaluating the source of excess nutrients in Goleta Slough.