Relating Inputs and Outputs: Pore-Pressure Responses to Natural and Anthropogenic Signals

Abstract:

Water levels are constantly changing due to natural (recharge, barometric pressure, tides, etc.) and anthropogenic (e.g. extraction/injection) forcings. Some of these changes may be small and/or short lived and therefore require monitoring at high precision and high frequency for interpretation. Traditionally, the evaluation of aquifer properties has relied on specific hydraulic tests that minimize the number and complexity of inputs to the system, which typically involve the removal or addition of known quantities of water (e.g. constant rate pumping tests, slug tests). These methods attempt to overcome background “noise” by generating a water level response (output) which is easily extractable from the water level record by using inputs that cause changes that are much larger than ambient conditions, or by using a unique frequency (sinusoidal tests). Instead of using single hydraulic tests, ambient monitoring of pore-pressure over multiple months can detect thousands of responses as the system is stressed by natural and anthropogenic forcings. Improvements to monitoring devices and methods help provide confidence in the mapping of the complex inputs to the subsurface pore-pressure responses. Signals previously treated as “noise” may now be interpreted with higher confidence due to improvements in data acquisition and processing. This study uses high frequency (every second) monitoring of pore-pressure responses to natural and anthropogenic signals at multiple depths in an actively managed aquifer. Using superposition in time and space, pore-pressure responses are deconvolved to give the relevant components of the system. Different volumes of the subsurface are tested by different signals, for example, barometric response may be dependent on vadose zone properties, however, the response to municipal pumping is relatively insensitive to the vadose zone. Therefore, the interpretation of ambient monitoring data can provide a multi-scale (both in time and space) understanding of the subsurface.