Question: A stream has been monitored weekly for a number of years, and the total dissolved solids in the stream averages 40 parts per million and is constant throughout the year. Following a recent change in land use in the drainage basins of the stream, a fluvial geo morphologist finds that the mean parts per million of dissolved solids in a 25-week sample to be 52 with a standard deviation of 32. Has there been a change in the average level of dissolved solids in this stream?