This thesis investigated the influence of septic tanks (ST) on stream water quality. Characterisation of septic tank effluent (STE) revealed that STE were enriched in phosphorus (P), nitrogen (N), organic matter (OM) and metals relative to stream waters and large proportions of these parameters were present in the soluble reactive forms. Human factors such as tank design, number of users, dishwasher use and infrequent desludging significantly (P<0.05) influenced effluent quality. ST that received roof runoff had reduced effluent retention time, while infrequent desludging was linked to increased OM, bacteria and P concentration in the effluent. Tanks that served larger numbers of people had elevated microbial abundance, P and N concentrations. Effluent composition was not consistent throughout the year, but exhibited similarity in warmer drier months that was different from effluent in colder wetter months for biological oxygen demand and heavy metals. Effluent attenuation in a test soil revealed that 14%-35% of P was attenuated by sorption processes, while saccharin was strongly attenuated by soil microbial degradation. The complex composition of STE reduced the ability of the test soil to adsorb P, as other substances in the effluent were competing for soil binding sites. Calculated P annual loadings from STE were 0.797 and 0.956 kgP/person/year for water usage of 150 and 180 l/person/day, respectively, while, effluents P load from detergents was 0.154 kg P/person/year. Newly developed tracing studies showed that ratios of chloride to other effluent indicators (e.g. EC, NH4-N, TSS, turbidity, total coliforms, sucralose, saccharin and Zn) and the detection of effluent tryptophan-like peak by fluorescence spectroscopy may be useful in tracking effluent discharge to streams with low levels of dilution. However, effluent caffeine and saccharin were more effective tracers in streams with low and high levels of dilutions. A single individual tracer alone was not sufficient to evaluate STE contamination sources, but combined chemical and physical tracing approaches show promise as tools to identify STE inputs that continue to pose risks to watercourses and where mitigation measures could be effectively targeted.