3.1 Water quality of total coliform numbers
Due to the fact that the sampling site B became drought and no sample could be collected after October 2019, the year-round water quality analysis was completed mostly from two of the three initial sampling sites: Site A and C, representing the North and South Pond, respectively (Fig. 1). The monthly total coliform indicated that the population was dramatically increased, during wet seasons in both of the two ponds. In the wet seasons, the population in Site C frequently exceeded 5,000 CFU 100 mL-1 (Fig. 2), which means Category B standard was violated in the South Pond (Table 1). Worthy being considered, a pattern about the coliform growth tends to be associated with the location, the pond capacity, and the flooding day. The highest number of the total coliform is often observed in Site C, which located downstream of the Site A. Also, the time series plot shows, since August 13 2019 (the flooding day), the highest coliform number was firstly observed on Site A, and then followed by Site B, and C; along with the flow direction from north to south. The pattern in 2019 revealed that the coliform number started to grow on the flooding day (August 13 2019) and then became gradually accumulated in Site B and C. Therefore, the highest coliform numbers detected in Site C can be explained: (1) during the flooding time, the South Pond might receive dirty water (with high levels of coliform) spilled from the North Pond and the SY River, and (2) due to the relative large sedimentation area in the South Pond, the concentration of the total coliform increased suddenly. When the flood water was not able to be captured by the South Pond, it leaked into the underground aquifer in a short period of time. Although flow capture efficiency was an important performance indicator of pollutant removal effectiveness of storm water detention ponds (Guo and Adams, 1999), the South Pond seems to have relatively short detection time, comparing with the North Pond.
To verify if that the increase of the total coliform is correlated with the flooding time, correlations between the coliform number and previous 7-day rainfall average were estimated (Fig. 3). The correlations were calculated between the total coliform numbers and moving averages of the past 7-day precipitation. The scatter plot and the correlation coefficient (rnorth(A): 0.946 and rsouth(C): 0.442) indicated significant association between the early precipitation and the later raised coliform numbers. Therefore, the peak flood occurred on August 13 2019 indeed caused the exceeding coliform growth. The water quality indicated by the total coliform number was seriously affected by rainfall, so the South Pond may not be recommended for leisure activity after a big rainfall is just observed.
A yearly distribution of the coliform detected in the two ponds is described with a box plot (Fig. 4). Perhaps due to a relative large sedimentation capacity, the water quality in South Pond (C) was not stable, revealed by a relatively large coliform population distribution. The interquartile range (IQR) between Site C and A was 1,392 and 372 CFU 100 ml-1, and the maximum count in Site C and A was 8,700 and 4,500 CFU 100 ml-1, respectively. Also, the water quality varied tremendously between the wet and dry seasons. To illustrate, the total coliform units grew tremendously in the summer flooding days but dropped to around 1,000 CFU 100 ml-1 in the other seasons.
3.2 Water quality indicated with the Surface Water Classification
To make a fare evaluation of the JDP water quality, we applied the Surface Water Classification and Water Quality Standards (TEPA, 2017) to make comparisons between the monitored results with the baseline values (Table 1). The associated parameters with the relevant environmental standards pertaining to protection to the living environment including DO, pH, SS, NH3-N, TP, and temperature were measured (Fig. 5). Some of them were not conducted until 2020.
The classifications of terrestrial surface water bodies are intended to specify the scope of application but not to restrict the uses, the relevant environmental standards were made pertaining to protecting living environment and human health (Table 1). Category A, B and C were selected in the classification specific to terrestrial surface water bodies such as rivers and lakes. Category A water bodies may be used for class 1 public water, which can be used for swimming pool after only several simple purification processes are completed. Category B water bodies may be used for Class 2 public, Class 1 aquaculture water, which associated with raising organism (trout, sweet fish and perch) involving food, restoration, conservation, or sport. Category C water bodies may be used for functions such as Class 3 public water and Class 2 aquaculture water. Because the water in the JDP is not designated for public use, neither for swimming or any full body contact activity, Category C was chosen as base line for the current application but Category B was the goal for primary-contact water leisure activities. Therefore, the water quality data are displayed with marked levels of Category A, B, and C (Fig. 5).
During the dry seasons, DO values detected were often greater than 130% of saturation based on the corresponding temperature in Site C (Fig. S2). The data implied a worse water quality. Although in natural water bodies, DO of over saturation could be observed due to water-dropping aeration or photosynthesis, over 130% of saturated DO is a symptom of deteriorated water quality (TEPA, 2010). Algal bloom is assumed to be observed following by DO of 130% over saturation. The 130% over saturated DO was detected in Site C during September - December in 2019 and June - July in 2020. Additionally, during September - December in 2019, pH was remarkably high (over 9.0) and beyond Category B. Additionally, SS in Site C hardly attained Category B. NH3-N in Site C attained Category B. TP cannot attain any category levels in both of the two ponds.
DO and NH3-N have been listed as critical pollutants to be controlled in the SY River watershed, due to the small business and urban residential water use upstream (TEPB, 2020; PCEC, 2019). In the watershed, pollution sources contribution analysis showed that 78.1% was caused by municipal waste, 15.8% from industrial, and 6.2% from livestock (TEPB, 2020). The high TP measured in the two ponds might be the result of incompletely treated effluent from the SY River. For example, detergent waste from the residential water discharged to the SY River could be one of the pollution sources. The observed high TP could cause an outbreak of eutrophication. TP levels was measured as 0.36±0.19 mg/L and 1.89±0.52 mg/L, in North and South Pond, respectively. Nearby the North Pond located the Wandai Water Purification Plan and the Rende Drain Water Purification Plant, which were constructed for the purpose of maintaining water quality in the SY River. However, both of the water plans were designed with aerated gravel-packed contact beds, which were useful in treating SS, BOD, and NH3-N, but not in controlling TP. TP removal is recommended to put into consideration for better water quality in JDP. The water quality in the confined water system could be deteriorated due to potential eutrophication, which would affect the expected water function including leisure amusement. To avoid the eutrophication, an applicable TP control strategy could be necessary. Production of aquatic plant would be reduced as much as 50% in case of dissolved phosphorous can be reduced to around 0.025 mg L-1, which is about the level required with Category A (lower than 0.02 mg L-1) (Deevey, 1970). Also TP was recognized as a growth limiting factor of algae such as cyanobacteria, which is famous of forming harmful algal blooms. Controlled TP into water body was suggested as the most effective way to limit potential algal growth (Thomas, 1969). US EPA (2018) reported media installation as retrofit device in detention basins exhibited greater than 56% of phosphorous removal, but the cost is the consideration for the widespread use of media such as ferric oxide (the most expensive) and the switchgrass (the least cost).
An attainment percentage table was constructed to present a fare evaluation of the JDP water quality (Table 2). In North Pond, 100% of the total coliform, pH, SS, and NH3-N attained Category B, and 92% of DO observations attained Category C. In South Pond, only 44-100% among the total coliform, DO, pH, SS, and NH3-N achieved Category B and C. TP in the two ponds were not able to attain any of the categories. In brief, the South Pond revealed modest water quality, according to the elevated DO and pH during the dry seasons, as well as the exceedingly high total coliform numbers during the wet seasons. On the contrary, Site A at the North Pond displayed a normal DO variation during the same study time. North Pond is ready to be designed in the recreational park with the relatively good water quality. South Pond had problems of over saturated DO and TP exceedances. Although the total coliform violated Category B in the Surface Water Classification and Water Quality Standards (TEPA, 2017) during the flooding days, the level as the fecal indicator bacteria listed in the bacterial water quality standards did not prevent the pond from leisure activity (US EPA, 2003). As far as the water leisure activity does not involve swimming, the current monitored water quality is adequate as the reference base line of Category C was mostly attained.
3.3 Water indicated with the Bacterial Water Quality standard
According to US EPA (2003), fecal bacteria have been used as an indicator of potential presence of pathogens in surface water and risk of disease. Among the fecal bacteria, fecal coliforms, enterococci, and Escherichia coli, are used as the primary indicators. Although scientific advancement in the fields of microbiology, statistics, and epidemiology have emphasized that curturable enterococci and Escherichia coli have a higher degree of association with outbreaks of certain diseases than fecal coliforms and total coliforms (US EPA, 2012), only total coliform is listed in the Surface Water Classification and Water Quality Standards for us to make comparisons (TEPA, 2017). Therefore, we evaluated the JDP water quality also with the available bacterial water quality standards, which applied total coliforms in the certain EPA Regions (US EPA, 2003). The reviewed US EPA standards (2003) were scientifically related to protection of human health in waters designated for primary contact recreation, including swimming and surfing. For example, in New York State, no more than 20% of the total coliform samples may exceed 5,000 CFU 100 mL-1, and water bodies beyond the level cannot be considered recreational purposes. In Utah, the secondary contact recreational uses same as the primary contact level are 1,000 – 5,000 CFU 100 mL-1, designating certain waters as “swimmable.” In California, no sample allowed to exceed 10,000 CFU 100 mL-1 in recreational waters. These standards are defined as a concentration level above which the health risk from ingestion of contaminated surface water is unacceptable high. The waterborne diseases have been demonstrated with evidence of gastrointestinal disorders form ingestion of polluted surface water. Based on the above references, fecal contamination in the JDP was not an issue that can prevent it from being leisure water activity, since even the highest number during the flooding did not exceed 10,000 CFU 100 mL-1 (Category C in TEPA, 2017). The nearest wastewater treatment plant, Hu Wei Liao Water Resource Recycle Center, upstream of the JDP, performed a controlled discharge of indicators from fecal pollution. The discharges of indicators from fecal pollution were relatively stable, except during the flooding days.