No abstract

The water quality of the Mun River, one of the largest tributaries of the Mekong River and an important agricultural area in Thailand, is investigated to determine its status, identify spatiotemporal variations and distinguish the potential causes. Water quality dataset based on monitoring in the last two decades (1997–2017) from 21 monitoring sites distributed across the basin were analyzed using seasonal Kendall test and water quality index (WQI) method. The Kendall test shows significant declines in fecal coliform bacteria (FCB) and ammonia (NH3) in the upper reaches and increases in nitrate (NO3) and NH3 in the lower reaches. Strong temporal and spatial fluctuations were observed in both the concentrations of individual parameters and the WQI values. Seasonal variation of water quality was observed at each monitoring site. WQI values in August (flood season) were generally among the lowest, compared to other seasons. Spatially, sites in the upper reaches generally having lower WQI values than those in the lower reaches. Excessive phosphorus is the primary cause of water quality degradation in the upper reaches, while nitrogen is the primary parameter for water quality degradation in the lower reaches. Urban built-up land is an important “source” of water pollutants in the lower basin, while agricultural land plays a dual role, affecting across the basin.

This work studies O 3 pollution for Chon Buri city in the eastern region of Thailand, where O 3 has become an increased and serious concern in the last decade. It includes emission estimation and photochemical box modeling in support of investigating the underlying nature of O 3 formation over the city and the roles of precursors emitted from sources. The year 2006 was considered and two single-day episodes (January 29 and February 14) were chosen for simulations. It was found that, in the city, the industrial sector is the largest emissions contributor for every O 3 precursor (i.e., NO x , non-methane volatile organic compounds or NMVOC, and CO), followed by on-road mobile group. Fugitive NMVOC is relatively large, emitted mainly from oil refineries and tank farms. Simulated results acceptably agree with observations for daily maximum O 3 level in both episodes and evidently indicate the VOCsensitive regime for O 3 formation. This regime is also substantiated by morning NMVOC/NO x ratios observed in the city. The corresponding O 3 isopleth diagrams suggest NMVOC control alone to lower elevated O 3 . In seeking a potential O 3 control strategy for the city, a combination of brute-force sensitivity tests, an experimental design, statistical modeling, and cost optimization was employed. A number of emission subgroups were found to significantly contribute to O 3 formation, based on the February 14 episode, for example, oil refinery (fugitive), tank farm (fugitive), passenger car (gasoline), and motorcycle (gasoline). But the costeffective strategy suggests control only on the first two subgroups to meet the standard. The cost of implementing the strategy was estimated and found to be small (only 0.2%) compared to the gross provincial product generated by the entire province where the city is located within. These findings could be useful as a needed guideline to support O 3 management for the city.Implications: Elevated O 3 in the urban and industrial city of Chon Buri needs better understanding of the problem and technical guidelines for its management. With a city-specific emission inventory and air quality modeling, O 3 formation was found to be VOC sensitive, and a cost-effective control strategy developed highlights fugitive emissions from the industrial sector to be controlled.