Faculty members from the Biology Department at UIN Sunan Kalijaga Yogyakarta, Siti Aisah, M.Si., and Ardyan Pramudya Kurniawan, S.Si., M.Si., are involved in a study on the pollution load of the Gajahwong River. Gajahwong is one of the three main rivers that traverse Yogyakarta. It is a sub-watershed (Sub Daerah Aliran Sungai or DAS) within the Opak Watershed (DAS Opak). Apart from the Gajahwong sub-watershed, the Opak Watershed also receives water from the Winongo River and the Code River.

The Gajahwong River is divided into two water quality classes: Class I and Class II. The upper reaches fall into Class I and are used as raw material for drinking water, which requires prior treatment. The middle to lower reaches fall into Class II and are used for recreational water facilities, fish farming, livestock, and agricultural irrigation. It is suspected that household and industrial wastewater flowing into Gajahwong contributes to pollution and a decline in water quality. Gajahwong has been exposed to both point-source and non-point-source pollution. The study of pollution load involves measuring the amount of pollutants present in the water. Mathematically, pollution load is defined as the mass of pollutants per unit of time (e.g., expressed in kg/day).

The study yielded the following conclusions:

• The sources of pollution in the Gajahwong River within the Yogyakarta city segment can be categorized into two types: point sources and non-point sources. Point sources include hotels and residential areas, hospitals and clinics, restaurants, and industries. Non-point sources include domestic waste and garbage. The pollution load in the Gajahwong River is predominantly contributed by the household sector.
• Water quality, as indicated by the Biological Oxygen Demand (BOD) parameter, mostly exceeds the Class II water quality standard in the downstream segment of the Gajahwong River within Yogyakarta city. There is a decline in water quality from the upper to lower reaches. However, the Total Suspended Solids (TSS) and chromium (Cr) parameters still meet the standard.
• The pollution load for BOD in the Gajahwong River is 369.58 kg/day, with the largest contribution coming from the domestic sector. The TSS pollution load is 21,990.528 kg/day, and the Cr pollution load is 28.7 kg/day.
• The carrying capacity for TSS and Cr can still accommodate pollution loads, but for the BOD parameter, several sub-districts are unable to handle the pollution load. Specifically, the sub-districts of Baciro, Demangan, Muja-muju, Pandeyan, Rejowinangun, Prenggan, and Purbayan are affected.

Recommended program activities based on the study findings include:

• Implementing pollution reduction programs, especially for BOD pollution. Existing communal wastewater treatment plants (IPAL) need effective monitoring to reduce pollution loads.
• Selectively granting permits for activities in river segments that have exceeded their pollution load-carrying capacity.
• Exploring the potential of a pollution load quota system with associated fees based on damage or emission charges.
• Given the significant contribution of the domestic sector to Gajahwong River pollution, public awareness campaigns and increased knowledge about pollution sources and household-level management are essential.
• While the QUAL2Kw method is acceptable for modeling, it is limited to a specific set of monitoring data. To improve modeling accuracy, future studies should focus on data exploration infrastructure for river systems.