Alteration of the groundwater thermal regime caused by advection

ABSTRACT

Groundwater temperature at an arbitrary depth and at an arbitrary point is determined not only by heat transported by conduction but also by advection caused either by infiltration of rain, snowmelt or irrigated water, or by seepage from surface water bodies. Therefore, characteristic changes of groundwater temperature are observed in recharging and discharging areas within a groundwater flow system. The changes may be one-, two-, or three-dimensional, depending on individual situations. Since heat is a conservative quantity in the subsurface environment, groundwater temperature can be used as a tracer to reveal the regional structure of a groundwater flow system. A case study showing the importance of groundwater temperature in a regional groundwater survey is presented taking Nagaoka plain, Japan, as an example. The groundwater temperatures were measured in observation wells with diameters of 65 to 250 mm and depths of 20 m or more. Marked seasonal changes in temperature depth profiles showing advective effects in the horizontal direction from the Shinano River, and in the vertical direction from upper and lower aquifers, were observed. The temperature depth profiles were classified into six types. The distribution of these types does not contradict the regional structure of the groundwater flow system revealed by the potential distribution. As groundwater temperature is an easily measureable element in a hydrological survey, the method described in the present paper is appropriate for a field study in an uninstrumented groundwater basin.     

Mathematical analysis to discriminate two types of sandstone-shale alternations

Sedimentation processes forming a series of bipartite layers have been studied mathematically. Two types of sedimentation processes are recognized, i.e. concurrent deposition of sandstone and shale by a turbidity current (Type I) and alternate deposition of each of them (Type II). A time series of events in a sedimentation process is reasonably considered a first-order Markov chain, and the process is described with a Markov matrix including four state-variables such as deposition of sandstone, erosion of sandstone, deposition of shale, and erosion of shale. Analysis of Markov matrix yields a fixed probability vector, which for Type I process is different from that for Type II process. The vector bears a close relation to Kolmogorov's coefficient, which is the ratio of the number of beds deposited and the number of beds preserved in a given sedimentary section. This coefficient can be computed on the basis of field observations. Substitution of the computed data determined the values of the fixed probability vector for two sedimentary sections in Japan. The results permitted a theoretical conclusion as to the genesis of observed sandstone-shale alternations. This conclusion is in good agreement with the deductions from more conventional sedimentological methods.