Spatial and temporal analysis of water quality from 1991 to 2011 in Miyun Reservoir

Cluster analysis, discriminant analysis-DA, and principal component analysis/factor analysis were used to analyze temporal-spatial variations and sources of water quality from 1991 to 2011 in the Miyun Reservoir. Water quality analysis was conducted in three interannual (IA) groups: IA I (1991-1993, 1995), IA II (1994, 1996-2000, 2002-2006), and IA III (2001, 2007-2011); two seasonal clusters: non-flood season (NF, November-December and January-April) and flood season (FL, May-October); and two spatial clusters (MP-main part of Kuxi and Kudong and NH-Neihu), based on spatial I (I-Kudong, I-Kuxi, and I-Neihu), spatial II (II-Kudong, II-Kuxi, and II-Neihu), spatial III (III-Kudong, III-Kuxi, and III-Neihu), spatial NF (NF-Kudong, NF-Kuxi, and NF-Neihu), and spatial FL (FL-Kudong, FL-Kuxi, and FL-Neihu). Spatial variations between MP and NH were lower than those between seasonal and IA variations. IA DA showed that electrical conductivity (EC), Ca²⁺, Mg²⁺, T-Hard, and T-Alk were due to carbonate dissolution accelerated by SO₄^2- both from fertilizer use and industrial activities. Effective control measures decreased BOD₅ and NO₃^--N. Spatial variations at spatial IA and seasonal scales showed that high levels of significant parameters in MP were mostly attributed to non-point pollution from watershed, whereas cage culture and sediment release in NH. The main pollution was comprised of nitrogen, phosphorus, organic, and other ion pollutants (Ca²⁺, SO₄^2-, Mg²⁺, T-Alk, EC, and T-Hard). Future studies must focus on water circulation enhancement, timely sediment dredging, and decreasing non-point pollution in FL (water and soil loss, fertilizer use, and cage culture) and anthropogenic discharge in NF.