Streams are important parts of our ecosystem and crucial water sources. Knowing composition of stream waters helps to understand where it came from and how it can be used. Geochemists test stream water samples using different methods to see the spread of elements and ions across the country. 16 different elements and properties were mapped on a 1:100,000 scale (1cm on the map relates to a distance of 1km). A total of 6836 stream water samples were collected between 2011 and 2017. Samples were taken from evenly spread smaller or medium sized streams. The data measures changes in the strength of a number of elements in two ways. The types of elements can point to what rocks the water had contact with. Their parent material is called as a source rock. Also tested were conductivity, which tells us the amount of dissolved ions are in the stream water, pH, which shows how acid or basic the waters are, alkalinity which is tells us how hard the water is and major ions which tells us what major dissolved salts are present in the waters. This is a raster dataset. Data is presented in form of an image and colour scales are used to show the different strengths of the elements. Raster data stores information in a cell-based manner and consists of a matrix of cells (or pixels) organised into rows and columns. The format of the raster is a grid. The grid cell size is 250 m which means that each cell (pixel) represents an area on the ground that is 250 meters across. The grid also contains location information. The Tellus survey is a national airborne geophysical and ground geochemical mapping project managed by the Geological Survey Ireland.