FRACTURE FLOW DYNAMICS INTO A WATER SUPPLY TUNNEL, SANTA BARBARA, CA, BASED ON GEOCHEMISTRY AND A 23 YEAR RECORD OF SEEPAGE FLOW

The Mission Tunnel was constructed between 1904 and 1912 as a water supply conduit from the Gibraltar reservoir to the city of Santa Barbara. The tunnel extends 6 km through a nearly vertical dipping, fractured sequence of sandstones and shales that make up the Santa Ynez Mountains. Stable isotope data of waters seeping into the tunnel suggest that groundwater flow is nearly vertical through fractures and along bedding planes. This geologic setting and the historical data sets of precipitation, runoff, and groundwater seepage into the tunnel provide an interesting look at fractured flow.

Spectral analysis of the nearly 25 year record of precipitation, runoff, and groundwater flow into the Mission Tunnel indicates a strong annual frequency. Although there is a good correlation between the amount of groundwater seepage and the amount of precipitation during a given year, the energy of the trace for these records suggests that the heaviest precipitation events do not always lead to the largest seepage events. This is likely to be due, at least in part, to the intensity of the storms and the amount of runoff and infiltration that is produced.

The frequency spectra of the record of groundwater seepage into the tunnel also shows a strong 4 year cycle suggesting that the antecedent conditions have an affect on the amount of seepage reaching the tunnel. In addition, the strength of these cycles fades during multiple consecutive dry years. The maximum correlation between precipitation and groundwater seepage was also calculated and shows that there is an offset between the two records in which groundwater seepage lags precipitation by one to three months, depending on whether precipitation was above or below average over the period. This lag is likely to be representative of the time necessary to wet the previously dry portions of the fractures sufficiently to produce increased groundwater flow.

Geochemical age tracers, including chlorofluorocarbons (CFCs) and tritium, suggest that groundwaters are likely to be a mixture of >50 year old water and nearly modern waters. This suggests that while fast paths caused by fractures are likely to dominate the flow system during wet times, matrix flow and slower travel paths also contribute significantly to the flow reaching the tunnel.