Temporal and Spatial Variations in Total Suspended and Dissolved Solids in the Upper Part of Manoa Stream, Hawaii

Abstract

Hawaiian watersheds are small, steep, and receive high intensity rainfall events of non-uniform distribution. These geographic and weather patterns result in flashy streams of strongly variable water quality even within various stream segments. Total suspended solids (TSS) and total dissolved solids (TDS) were used to investigate the variability in water quality in the upper part of Manoa Stream in Honolulu, Hawaii. With a few interruptions, water samples were taken on a daily basis between September 2005 and June 2006. The samples were analyzed for TSS and TDS, and varied from almost 0 to 724 and to 302 mg L-1, respectively. During the raining season (October through March) TSS and TDS were more variable, and TSS was higher than in the dry season (April through June). No relation was observed between TSS and TDS and discharge. This may be explained by the heterogeneous rainfall distribution which causes varying contributions from different sources. During one rainfall event TSS and TDS also varied considerably in time. Both TSS and TDS showed increasing trends going downstream suggesting that the urbanized area generates more suspended and dissolved matter than the forested conservation area upstream. However, given the large variability in TSS and TDS, the increasing trend downstream is associated with high uncertainty. The results of this study stress the necessity of recognizing the variability in water quality of small streams for setting up a monitoring strategy, adopting a modeling approach to predict water quality or extrapolating data from limited samples to annual loads in coastal regions.