Trends in surface water chemistry are evaluated regularly and reported in trend reports. The trend assesments show that surface water quality in acid-sensitive areas has improved strongly in Europe and North America since the 1980s. The consistent pattern of chemical recovery (decreasing sulphate and increasing pH and alkalinity) across a large number of sites is the strongest evidence that emission control programs are having their intended effect. Monitoring data from the ICP Waters programme document that the reductions in acid deposition are mirrored in improved water quality in most regions. In many areas, water quality now is sufficient for the return of acid-sensitive organisms such as fish, insects and shells.

The most important findings of the trend assessments are:

• There is an almost universal decrease in sulphate concentrations in lakes and streams throughout Europe and North America. In almost all cases the decreases in the 1990s are larger than in the 1980s, while the trends since 2000 have levelled off.
• Most sites show no significant trend in nitrate. In the last assessment (2004) roughly two thirds of all sites showed no trend in NO3, whereas about one fourth of all sites showed a decreasing trend. NO3 increased in less than 10% of the sites. The lack of a uniform trend in NO3 concentrations illustrates that NO3 leaching from catchment is affected by a variety of processes.
• This is in contrast to SO4 leaching from catchment, which is largely controlled by SO4 deposition. 
  Alkalinity and acid neutralizing capacity showed increasing trends at roughly one third of the sites while hardly any sites showed decreases in these variables. This indicated that virtually no sites were currently acidifying further, compared to 14% of the sites showing a decrease in alkalinity in 1990–2001. 
• Dissolved organic carbon (DOC) measurements were not available at all sites. About 80% of sites lacked a DOC trend, while the remaining showed a significant increase. The number of sites with increasing trends in DOC has halved compared to 1990–2001. 

The conclusions from the ICP Waters programme are that observed chemical recovery is associated with declining deposition of sulphate. However, despite reduced sulphate deposition, some regions show no reductions in sulphate concentrations in surface waters and no improved water quality. Clear examples are the Blue Ridge Mountains in Viriginia (USA) and the Harz mountains in Germany. Here, deep soils with a large sulphate-adsorption capacity dominate and sulphate concentrations in surface water are controlled by desorption of S that was deposited in the past decades.