max-width:100%;height:auto;
Code:
@media only screen and (min-width: 769px) {
.gridContainer {
width: 1000px;
max-width: 1232px;
margin: auto;
}
is why that is happening.
by changing the width to 100% this will avoid the scroll appearing, but you'll have some other design issues to contend with as it's now a flexible layout. these issues will affect the header, ie <div id="header">and all it's children.
justified text is harder to read, and if you increase the line height, and probably the font-size a little more it'll be a lot easier to read.