SOIL-TEMPERATURE REGIME IN MIXED PRAIRIE RANGELAND AFTER PIPELINE CONSTRUCTION AND OPERATION

Pipeline construction and operation affect the soil thermal regime of the disturbed area by: (1) alteration of soil texture, bulk density, a nd water content and thus alteration of soil thermal properties, and ( 2) heat from the compressed fluid flowing through the pipe. Soil tempe ratures over pipelines may also be influenced by sparse vegetative cov er as a result of pipeline construction if revegetation is not success ful, in which case near-surface temperatures would be affected; or by heat from the pipeline, in which case subsurface soil temperatures wou ld be affected. These effects could alter plant growth in the vicinity of the pipeline. A study was conducted in Solonetzic mixed prairie ra ngeland of southern Alberta to evaluate soil temperature responses to pipeline construction and operation. Specific objectives were to quant ify these responses and assess their vertical and lateral extent on ad jacent natural gas pipelines, installed in 1957 and 1981, with diamete rs of 0.86 and 1.07 m, respectively. Soil temperatures were monitored throughout the year to a depth of 1.10 m directly over he two pipeline s, adjacent to the 1981 trench at distances of 3.0 (pipelay area) and 10.0 m (work area) and in the adjacent undisturbed native prairie. Pip eline construction and operation in Solonetzic mixed prairie rangeland caused greater maxima in, and more pronounced fluctuations of, near-s urface soil temperature compared to the undisturbed prairie. Soil temp eratures at depths up to 0.80 m above the 1981 pipe, and to at least 3 m laterally from it, increased during the winter months, while temper atures during the summer were cooler in the 1981 trench. The 0-degrees -C isotherm was much shallower over winter in the 1981 trench than in all other monitoring locations. Differences in the thermal regime betw een the 1981 and 1957 pipelines could not be attributed to any single cause due to a complicated interaction of factors and different reclam ation practices used following installation of the two pipelines. Cons truction traffic altered the thermal regime of the work area. Mid-summ er near-surface temperatures were high at all locations, less affected by pipeline construction and operation and likely limiting to plant g rowth.